typo error :  in one line

Textbox = FirstName & vbnewline & LastName

Will Show up as

FirstName
LastName

thanks for your reply

but...  it was not working, when I checked the property box , it appeared  [Firstname] & [vbnewline] & [Lastname] ;so I changed to [Firstname] & "vbnewline" & [Lastname], it appeared FirstnamevbnewlineLastname

That's VBA code, put it in the on current event of the form.

If you want to do it in the control source property, you'd need to do something like
=[FirstName] & Chr(13) & Chr(10) & [LastName]

done . thanks Adam