Polymeric glasses are important engineering materials in industry application and they are also of great academic interest. Unlike non-polymeric glasses rarely yielding during the external deformation, polymeric glasses with high molecular weight can undergo various mechanical response, such as shear yielding, necking, crazing, strain hardening, and brittle failure. Although enormous efforts, dating back to 70 years ago, have been made to understand the physics behind, the nature of the various mechanical response is still unclear and under debate. In this dissertation, we combine mechanical tests with computer simulation to investigate the mechanical response of polymeric glasses. With the help of our recently proposed hybrid structure model, we are able to understand the brittle-ductile transition, origin of stress, relaxation and elastic yielding. It is found that chain network is essential for polymeric glass system to avoid brittle failure. However, the failure of specimen with high molecular weight can also be achieved by dynamically chain pull-out through crazing. In the case that failure is avoided and large deformation is achieved, chain network is essential for building high level of stress, no matter in tensile or compression mode. When specimen relaxes after large deformation, relaxation of chain network is independent of alpha process, therefore is able to hold stress at temperature near Tg. Otherwise, if the specimen is released after large deformation, it is found that the imbedded stress coming from strained chain network, which is able to show up after increasing temperature.