   How to use D3DXConvertMeshSubsetToSingleStrip Function??
  I called it like this...D3DXConvertMeshSubsetToSingleStrip(g_pMesh,0,D3DXMESH_MANAGED ,&pIndexBuffe,&NumIndices);But it asked me D3DERR_INVALIDCALL...... 
  Maybe something is wrong with your original mesh.  Are you using the Debug Runtimes?  If so, what does the debug spew say?  You should also make sure that the mesh is valid by calling D3DXValidMesh and D3DXCleanMesh.neneboricua 