libpq.so.3 not found

Hi,all
When I try to create an executable file using gcc -o -L -lpq, it gives me
an error message:
ld.so.1: file_name: fatal: libpq.so.3: open failed: No such file or
directory
But I give the full path of lib files in -L option. And libpq.so.3 is
right there.
Can anyone help me?
Thanks a lot.
Xinyu