Lap shear and friction force measurements were carried out on a series of monodisperse polystyrene (PS) films below the corresponding glass-transition temperatures. It showed that adhesion between the PS/PS interface was possible at the temperature below the bulk T-g, and the lower the molecular weight of PS, the lower the temperature at which the interfacial strength was detectable. The examination of a series of molecular weights indicated both the surface molecular motion and the magnitude of the interfacial strength were dependent on molecular weight and its distribution. And a steep increase of the friction force with increasing the test temperature was observed around 0 similar to 30 degrees C. The contact angle of water versus molecular weight measurements also showed a transition at room temperature. The behavior observed in this study was supposed to be due to the increased molecular mobility, and was in good agreement with the measured surface transition temperatures by DSC.