1. Render 100 meshes, FPS:42 2. SetNumFaces() to 1/4 of original, Render 100 of them, FPS:42 No effect?? My PC is P4 2.4A + Radeon 9550, 1G RAM, SDK version is April 2006

There are a lot of factors at play here. First, make sure that your LOD call actually succeeded and isn't failing or something. How many verts/tri's did a mesh have to begin with? It is likely that you are simply not throwing enough at your card to make an even noticable dent in performance. The low framerate could be coming from a limit in batching or the shaders.

I watched the render result, the faces are indeed reduced. I just wondering why the fps doesn't change.
Maybe I'll try to reduce the number of beginXXX endXXX calls...