How can I delete all topics in one of my forums? (I don't want to delete that forum, I just want to delete all of it's topics. It has about 1200 topics)

Perhaps this is an option for you: delete the forum (so that you can delete all posts/threads in it) and then re-create it again.
-mb

marsbar
If I do this the forum number is change, and all of my previous setting (define moderators and ...) must be change

I think, it's possible to do only directly in database. For example, by the following command:

delete from minibbtable_topics where forum_id=FORUM_ID;
delete from minibbtable_posts where forum_id=FORUM_ID;

But please note: number of postings per each users also needs to be recalculated. I recommend to run Update Script (convdb.php) after all and choose "second" option to re-calculate amount of topics and posts in static fields.

If you have large amount of posts, it's worth to recalculate only number of user postings (not also topic latest post time, as this script does by default). You may comment out the algorithm which states as /* Topic last post time */ and run this script calculating only user postings then.

Paul
Thank you so much ;-)