Atactic-polystyrene (a-PS) films cast from solutions of different concentrations in dichloromethane under vacuum conditions were prepared．Their tensile mechanical behavior has been studied．It was found that the tensile strength of films cast in vacuum increases significantly with the solution concentration when the solution concentration is above 0.02 g／mL．The existence of a critical concentration value in the region of 0.01～O.02 g/mL is in agreement with the results of previous experiments．The critical concentration has been identified as ce for coil entanglement．