In order to increase load capacity and reduce weight, size and vibration level of gear transmission system, a new-style unsymmetric gear with different pressure angles was designed. Modification of pinion geometry could reduce the transmission errors and achieve a more favorable shape for the function of transmission errors. The process of meshing and contact for unsymmetric gear with double pressure angles in case of modified geometry and transmission errors was proposed. Computer programs of tooth contact analysis (TCA) of unsymmetric gear and conventional gear were developed. Two versions of modified geometry were investigated and compared with numerical examples. The results showed that the displacement of contact path of unsymmetric gear was smaller than conventional symmetric gear where alignment errors exist，and the function of transmission errors for unsymmetric gear was indeed a continuous parabolic one with a reduction in magnitude. It was proved that involute gear with unsymmetric tooth could efficiently improve transmission stabilization and load capacity of tooth surface.