diff --git a/pkg/composer/escpos_composer_test.go b/pkg/composer/escpos_composer_test.go
new file mode 100644
index 0000000..783c58f
--- /dev/null
+++ b/pkg/composer/escpos_composer_test.go
@@ -0,0 +1,160 @@
+package composer
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// ============================================================================
+// Mocks
+// ============================================================================
+
+type mockPrintCapability struct {
+	textFn                     func(string) ([]byte, error)
+	printAndLineFeedFn         func() []byte
+	printAndCarriageReturnFn   func() []byte
+	formFeedFn                 func() []byte
+	printAndFeedPaperFn        func(byte) []byte
+	printAndFeedLinesFn        func(byte) []byte
+	printAndReverseFeedFn      func(byte) ([]byte, error)
+	printAndReverseFeedLinesFn func(byte) ([]byte, error)
+	printDataInPageModeFn      func() []byte
+	cancelDataFn               func() []byte
+}
+
+func (m *mockPrintCapability) Text(text string) ([]byte, error) {
+	if m.textFn != nil {
+		return m.textFn(text)
+	}
+	return nil, nil
+}
+
+func (m *mockPrintCapability) PrintAndLineFeed() []byte {
+	if m.printAndLineFeedFn != nil {
+		return m.printAndLineFeedFn()
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) PrintAndCarriageReturn() []byte {
+	if m.printAndCarriageReturnFn != nil {
+		return m.printAndCarriageReturnFn()
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) FormFeed() []byte {
+	if m.formFeedFn != nil {
+		return m.formFeedFn()
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) PrintAndFeedPaper(units byte) []byte {
+	if m.printAndFeedPaperFn != nil {
+		return m.printAndFeedPaperFn(units)
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) PrintAndFeedLines(lines byte) []byte {
+	if m.printAndFeedLinesFn != nil {
+		return m.printAndFeedLinesFn(lines)
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) PrintAndReverseFeed(units byte) ([]byte, error) {
+	if m.printAndReverseFeedFn != nil {
+		return m.printAndReverseFeedFn(units)
+	}
+	return nil, nil
+}
+
+func (m *mockPrintCapability) PrintAndReverseFeedLines(lines byte) ([]byte, error) {
+	if m.printAndReverseFeedLinesFn != nil {
+		return m.printAndReverseFeedLinesFn(lines)
+	}
+	return nil, nil
+}
+
+func (m *mockPrintCapability) PrintDataInPageMode() []byte {
+	if m.printDataInPageModeFn != nil {
+		return m.printDataInPageModeFn()
+	}
+	return nil
+}
+
+func (m *mockPrintCapability) CancelData() []byte {
+	if m.cancelDataFn != nil {
+		return m.cancelDataFn()
+	}
+	return nil
+}
+
+// ============================================================================
+// Tests
+// ============================================================================
+
+func TestEscposProtocol_PrintLn(t *testing.T) {
+	tests := []struct {
+		name          string
+		input         string
+		mockText      func(string) ([]byte, error)
+		mockPrintFeed func() []byte
+		want          []byte
+		wantErr       bool
+	}{
+		{
+			name:  "success",
+			input: "Hello World",
+			mockText: func(s string) ([]byte, error) {
+				return []byte(s), nil
+			},
+			mockPrintFeed: func() []byte {
+				return []byte{0x0A}
+			},
+			want:    []byte("Hello World\n"),
+			wantErr: false,
+		},
+		{
+			name:  "text error",
+			input: "Error",
+			mockText: func(s string) ([]byte, error) {
+				return nil, errors.New("mock error")
+			},
+			mockPrintFeed: func() []byte {
+				return []byte{0x0A}
+			},
+			want:    nil,
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Setup mock
+			mock := &mockPrintCapability{
+				textFn:             tt.mockText,
+				printAndLineFeedFn: tt.mockPrintFeed,
+			}
+
+			// Create protocol with mock
+			c := &EscposProtocol{
+				Print: mock,
+			}
+
+			got, err := c.PrintLn(tt.input)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("PrintLn() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if !tt.wantErr && !bytes.Equal(got, tt.want) {
+				t.Errorf("PrintLn() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}