keil output "C51 FATAL-ERROR" in output windows when I build my target in a project, but it's right in another project! why? it is same file!
what is the exact error message? There is more information for a FATAL ERROR.
You can find a list of the Keil knowledge base articles here:
http://www.keil.com/c51/faq.asp
The most common FATAL ERROR warnings are listed in these articles, as well as ways to fix the error.
Open the page, and use your web browser to search for the words 'FATAL ERROR'