Executing Word Mail Merge

If your Word document is already configured with the merge fields, and you are running the macro from the workbook that contains the data you want to merge into the Word document, then try this:

Sub RunMerge()

    Dim wd As Object
    Dim wdocSource As Object

    Dim strWorkbookName As String

    On Error Resume Next
    Set wd = GetObject(, "Word.Application")
    If wd Is Nothing Then
        Set wd = CreateObject("Word.Application")
    End If
    On Error GoTo 0

    Set wdocSource = wd.Documents.Open("c:\test\WordMerge.docx")

    strWorkbookName = ThisWorkbook.Path & "\" & ThisWorkbook.Name

    wdocSource.MailMerge.MainDocumentType = wdFormLetters

    wdocSource.MailMerge.OpenDataSource _
            Name:=strWorkbookName, _
            AddToRecentFiles:=False, _
            Revert:=False, _
            Format:=wdOpenFormatAuto, _
            Connection:="Data Source=" & strWorkbookName & ";Mode=Read", _
            SQLStatement:="SELECT * FROM `Sheet1$`"

    With wdocSource.MailMerge
        .Destination = wdSendToNewDocument
        .SuppressBlankLines = True
        With .DataSource
            .FirstRecord = wdDefaultFirstRecord
            .LastRecord = wdDefaultLastRecord
        End With
        .Execute Pause:=False
    End With

    wd.Visible = True
    wdocSource.Close SaveChanges:=False

    Set wdocSource = Nothing
    Set wd = Nothing

End Sub

To get dendarii's solution to work I had to declare Word constants in Excel VBA as follows:

' Word constants
Const wdFormLetters = 0, wdOpenFormatAuto = 0
Const wdSendToNewDocument = 0, wdDefaultFirstRecord = 1, wdDefaultLastRecord = -16

If your word document is already configured with data source and merge fields layout then it becomes much simpler. In the example below MailMergeLayout.doc is all setup ready to perform a merge. A button in Excel is linked to RunMailMerge() as below. All the code is contained in an Excel VBA module.

Sub RunMailMerge()

    Dim wdOutputName, wdInputName As String
    wdOutputName = ThisWorkbook.Path & "\Reminder Letters " & Format(Date, "d mmm yyyy")
    wdInputName = ThisWorkbook.Path & "\MailMergeLayout.doc"

    ' open the mail merge layout file
    Dim wdDoc As Object
    Set wdDoc = GetObject(wdInputName, "Word.document")
    wdDoc.Application.Visible = True

    With wdDoc.MailMerge
         .MainDocumentType = wdFormLetters
         .Destination = wdSendToNewDocument
         .SuppressBlankLines = True
         .Execute Pause:=False
    End With

    ' show and save output file
    wdDoc.Application.Visible = True
    wdDoc.Application.ActiveDocument.SaveAs wdOutputName

    ' cleanup
    wdDoc.Close SaveChanges:=False
    Set wdDoc = Nothing

End Sub