using Word = Microsoft.Office.Interop.Word;

Word.Application wordApp = new Word.Application();
Word.Document doc = null;

// open the template file
doc = wordApp.Documents.Open(templateFileName, Type.Missing, bReadOnly);
doc.MailMerge.OpenDataSource(dataSourceFileName);
// Perform mail merge.
doc.MailMerge.Destination = Word.WdMailMergeDestination.wdSendToNewDocument;
doc.MailMerge.Execute();