I have got a strange shadow on my obj model (picture's right), but when I put a light pointing upwards from bottom, the shadow don't appear (picture's left). I saw some message say this was caused by inverted normal. But this was not happen in the modeler, only in poser. If those polygons really had inverted normal, should I saw a hole there, or a dark shadow like this? How can I make it correct ? Thx!

for an inverted normal, you would likely see a hole or a big black spot. Plus, in your modeller, you can have an inverted normal and still see it if its a forced 2 sided material. In this case, it looks more like you have an unwelded veritice somewhere in that general area or a misaligned face that is very dificult to see in the modeler but appears when you render in poser ( do to such a smaller scale usually) There was an earlier post in the forums with someone who had the same problem a while back, and he managed to fix it, I'll see if I can find it.

WHat it looks like to me is something I would expect. Take a look at the left image. The shadow is still there although less defined. It looks like the 'edge' actually bends up and that shadowed area is actually part of the underside. This is supported by the right image which shows the bend of the edge and the deeper shadow on the underside. I suggest going back to your modeller and viewing this in polygon mode very closely. You may find a not so straight edge here.

I feel that I know what is causing it. Could you please post a lined wireframe image. I'll then use that to mark up what you need to do.

thx you all ! closer look at the dark part, and a wirefram image, hope someone give me some idea

Try drawing in the white vertices then deleting the blue.

Yep Phil, we both saw the same thing. Trying to explain it is hard, glad you got the picture to show it.

Yep - Phil is right. This is a triangulation problem, and one of the reasons some people avoid triangulated meshes. There is a cool feature in MilkShape 3D that lets you select a triangle pair and automatically "flip" their common side - so you can see which variation gives the desired render. Triangles can be nasty to some rendering engines - others don't seem to care.