The pressure-broadened cross section (kinetic energy of the collision range:30 cm-1—1000cm-1) and the avarage relaxation cross section (temperature of the interaction system:125K,175K,300K) for pure rotational dipole transition of HCl in Ar are calculated,by employing infinite or-der sudden approximation (IOSA) and Ar-HCl H6(2) analyti potential energy surface of Hutson.Comparing the IOSA results with Close-Coupling results and experiments,we find that IOSA can give satisfactory results for j=0←j=1 transition line of the molecules which have large rotational constant.Unfortunately,modfication must be performed for using IOSA in the exitation of transition lines.