I can't login OEM 10G by web

Dear ,
I have created an 10g database(named OEM).
I can't login OracleEnterpriseManager 10g by web using account "system" both for sysdba, sysoper and normal.(without any error message!!). It just popup the "login database : OEM" again and again.
Pls help

PS.
1.the oracle enterprise manager 10g is running.
2.I can use the same account/password by sqlplus.