   Path finding On 3d mmorpg
  I want to do it on server side!  but that cast a lot! what should I do!   
  This probably belongs in the AI forum or possibly the networking forum.Your question is too broad. What have you attempted already that is too expensive? Have you Googled for "3d pathfinding" or something similar?   ;  search for A* algorithm its used a lot and pretty easy to implement   ;  I Know A* algorithm ,  but I think it cast a lot on server side! It seems that warcraft world do it on client side!   ;  Why would WoW do it client side.  All the AI for NPCs is handled on the servers... As far as I know pathfinding isn't needed for the client as you're moving it.  Collision detection maybe but I wouldn't trust the client for anything, ever.   ;  A* is indeed probably far too complex to be running server-side for every active unit in an MMO that could want pathfinding; especially since you probably don't have the server-farm-esque resources of an MMO like WoW to distribute your pathfinding tasks to.You may be better off with an edge walking algorithm, which is much less expensive as far as computational resources go (even though it will take a good deal of design effort to make the algorithm look nice, as edge walking has some caveats you must be aware of).   