This paper designs and implements OnlineFSCK (On-Line File System Checker), which checks the file system while it is online without modifying the in-kernel file system codes. The file system can continue providing service while it is being checked by OnlineFSCK. This paper proposes and implements an runtime imaging algorithm which, combined with the existing file system checkers, can provide us the ability to check the consistency of the existing running file systems. The prototype is implemented for ext3 file system. Experiments shows that our OnlineFSCK solution can guarantee the correctness as traditional file system checkers while keeping the performance of the production system neglectable impacted. As there is no need to modify the in-kernel codes to support it, Online FSCK can be easily extended to support other file systems.