We have 3 workflow on the Journal Entry, none of the workflow update postingperiod, However when we go on the journal entry, we see that a workflow has been called and update the postingperiod. We have over 75 workflows, and with action workflow, the guy before me, like workflow a lot, is there's a way to list all the fields that are affected in the workflow ? IF so, I can see whichone is updating the postingperiod.

it's possible that you have a workflow that is changing the date, and the date change automatically is changing the posting period. Do you have a sandbox? Unfortunately, if you don't know which workflow is causing it...i'd recommend disabling half of them in the sandbox and seeing it it made it stop. If not, disable the other half...then at least you narrowed it down. Then do that again until you figure out which workflow is causing your problem.