Thank you to all for giving reply to me .. anyway finally i resolve my issue ... Thanks.
Here i fallow the solution...
Dim rng As Microsoft.Office.Interop.Word.Range
Dim doc As Microsoft.Office.Interop.Word.Document
Dim headertext As String
For Each wordSection As Microsoft.Office.Interop.Word.Section In Travelerdoc.Sections
rng = wordSection.Headers(Microsoft.Office.Interop.Word.WdHeaderFooterIndex.wdHeaderFooterPrimary).Range
rng.Font.Size = 14
rng.InsertAfter(vbTab & vbTab & "WO : " & ds.Tables(0).Rows(0).Item("wo").ToString())
Next