A self-bonding strength development of ABS during hot plate welding was investigated by a lap-shear joint method. In the hot plate welding, the flow of molten ABS occurs and it is important factor of self-bonding strength development. The flow activation energies, E a obtained from relation of viscosity and temperature were 4–8 kcal mol −1 at different shear rates. The self-bonding strength increased with increasing time and temperature and its development at temperature range from 180 °C to 260 °C was found to follow a t 1/4 law based on the reptation model applied to interdiffusion at an amorphous polymer-polymer interface. In fracture of lap joint samples, shear failure and tensile failure were observed. In tensile failure, the crack-like flaw at the edge of bond was thought to act as a stress concentrator. Cracks propagated from it. The number and depth of plastically deformed ridges at the re-heated surface by shear were increased with welding time and temperature.