Autoadhesion strength of PS/PS interfaces in solvent welding was determined as a function of processing conditions by butt joint test. It was verified that the chain mobility and surface roughness at PS/PS interface were enhanced by the applied solvent having a similar solubility parameter as PS and resulted in the dramatic improvement of autuadhesion strength at PS/PS interface. It was found that the mechanism of solvent welding is dependent upon the chain mobility due to the diffusion of solvent to PS interface and the contact area at interface. When the welding temperature is lower than the boiling point of applied solvent, the effect of chain mobility on autoadhesion strength was dominated, while contanct area took more important role when welding temperature is above the boiling point of solvent, Autoadhesion strength increased with increasing contact time and contact temperature but the effect of solvent on autoadhesion strength became smaller.