Initially, it is an Excel file. Before I merge, I save it as a csv file because there are too many mergefields (more than 255) in my original Merge Excel File. If I save it as xslm, Word cannot merge it because it stops at 255 mergefields. That's why I save it as csv and merge. Is there a solution?

In that case, instead of:
strWorkbookName = ThisWorkbook.FullName
you should be populating strWorkbookName with whatever the name of your CSV file is. For example:
strWorkbookName = Split(ThisWorkbook.FullName,".xls")(0) & ".CSV"
Simply closing & reopening the workbook won't give you access to the additional fields.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]