Feng
Try this.
ALTER ROLLBACK SEGMENT <> SHRINK TO size; -IF OPTIMAL SIZE is defined , or TO MINEXTENT SIZE if optimal size is not defined.....Additionally SMON will try once every 12 hours to shrink all rollback segments that have grown
over the optimal

To shrink all rollback segments do the following

SET PAGES 0
TRIMS ON
ECHO OFF
VERIFY OFF
FEEDBACK OFF
TTITLE OFF
CLEAR COLUMN

SPOOL shrink_rbs.tmp

PROMPT SET ECHO ON
PROMPT SPOOL shrink_rbs.log
PROMPT

SELECT 'ALTER ROLLBACK SEGMENT ' || segment_name
|| ' SHRINK;'
FROM dba_rollback_segs
WHERE status = 'ONLINE'
/

PROMPT ALTER TABLESPACE rbs COALESCE
/
PROMPT SPOOL OFF
PROMPT SET ECHO OFF

SPOOL OFF
CLEAR COLUMN

SET PAGES 24
TRIMS ON
ECHO OFF
VERIFY ON
FEEDBACK ON


@shrink_rbs.tmp

PROMPT
PROMPT ******************************
PROMPT
PROMPT Output saved at shrink_rbs.log
PROMPT
Hope this help