use this namespace
Imports Word = Microsoft.Office.Interop.Word
Imports System.IO
code to compare
Private Sub CompareDocument()
Dim missing As Object = System.Reflection.Missing.Value
Dim readonlyobj As Object = False
Dim filename As Object = txtfile1.Text
Dim app As Word.Application = New Word.ApplicationClass()
Dim doc As Word.Document = app.Documents.Open(filename, missing, readonlyobj, missing, missing, missing, _
missing, missing, missing, missing, missing, missing, _
missing, missing, missing, missing)
Dim filenm As String = txtfile2.Text
Dim nm As String = txtpath.Text + "\" & getfilename(txtfile1.Text)
Dim filesavename As Object = nm
doc.SaveAs(filesavename, missing, missing, missing, missing, missing, _
missing, missing, missing, missing, missing, missing, _
missing, missing, missing, missing)
doc.TrackRevisions = True
doc.ShowRevisions = True
doc.PrintRevisions = True
doc.Compare(filenm, missing, missing, missing, missing, missing, _
missing, missing)
doc.Close(missing, missing, missing)
app.Quit(missing, missing, missing)
End Sub
Private Function getfilename(sfilename As String) As String
Dim fin As New FileInfo(sfilename)
Return fin.Name
End Function