Why is my mesh not all showing up when opened in unreal?

Hey there @sairomlee! It looks a little like those faces have reversed normals. If that’s the case, going into the material and enabling two sided rendering will show it how you’d like it! However this is just a bandaid, properly fixing the face normals in a 3D program is preferred as two sided rendering literally makes internal and external faces render, so you get a bit more overhead for every material that uses it.

If that’s not the case, there could be an import grouping issue. Let me know how it goes!