lim_nfs problem

hello:
I have some question when I use nfs on my own board.
but there is some error like this:

Root-NFS: Server returned error -13 while mounting /home/lim/Ramdisk
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or 02:00
Please append a correct "root=" boot option
Kernel panic: VFS: Unable to mount root fs on 02:00

But in my nfs server ,i can use mount -t nfs localhost:/home/lim/Ramdisk to everywhere i want
what is problem?