If the tape is frozen it will not be avail until unfrozen even if you expired all the images.

Have you run the report "images on tape" and verified all images are expired?

 As rightly suggested by Hinchcliffe, cehck if the media is frozen. If the media is no frozen, then run the command bpexpdate -deassignempty

After you expire the imge run a catalog backup which will delete the expired image i n the catalog as part of the cleanup.

Yongkang, uf the above suggestion don't work, what procedure did you use to expire the media?  (if you changed the "expiration date" associated with the media id via the gui, you may not have expired the images)

/usr/openv/netbackup/bin/admincmd/bpmedialist -m <mediaid>

/usr/openv/volmgr/bin/vmquery -m <mediaid>

check the media status whether it is frozen or not use command bpmedialist

if it is frozen unfreeze it using command bpmedia

if all the images r not expired and u no longer use them u can use bpexpdate -m mediaid -d 0 -host hostname -force [THIS COMMAND WILL EXPIRE THE TAPE N PUT IT INTO SCRATCH]

CHECK in reports whether any of the images in media is useful to u or not before running BPEXPDATE