hi, Chandra and JDoyle:

Thank your help !
Chandra, I compared tnsnames.ora in client side and server side, find unique different is tnsnames.ora of server using hostname in "address" section, but in client, it using ip address in "address" section. i must to be add alias of server machine to /etc/hosts of clients , then i can connected to database. but when i removed that alias and changed alias to ip address , same error occured. why? i must be resolve this problem.
JDoyle, i also renamed sqlnet.ora to other. but it also can't connected to my database.