ID3DXLine problem

i draw some line with ID3DXLine::DrawTransform(), found if the camera goes near the line, the line will not clip by system ! i have setup zn and zf, D3DXMatrixPerspectiveFovLH((D3DXMATRIX*)&mViewProj, fov_y, aspect, zn, zf); then, used mViewProj and mWorldView to DrawTransform(). the line is allright, but if camera move, clip is wrong... somebody can tell me how to fix this problem , thanks a lot