I experienced this situation.. I don't know exactly, but can you check your mongrel directory's name? Maybe It is not "mongrel-1.1.1-x86-mswin32-60" Just change the directory name to "mongrel-1.1.1-x86-mswin32-60" It works very well when you start mongrel server on command line But, while you are using the radrails IDE, It doesn't recognized the mongrel server where it installed nor.