A novel technique for the monostatic radar cross-section (RCS) computation of complex targets modelled with trimmed nonuniform rational B-spline (NURBS) surfaces is proposed using physical optics (PO). The stationary phase method (SPM) is demonstrated to be invalid in evaluation of the PO integral over trimmed surfaces. Thus a new method, which combines SPM with the Gordon method (namely SPM-Gordon) is presented as a substitute and good candidate. Examples show that excellent accuracy is obtained.