At tuning PID controller,ultimate gain Ku and ultimate period Tu of the proceseare usually the desired parameters.To lessen disturbance of tuning action and av oid to drive thesvstem to the verge of instability,a new estimating methed of Ku and Tu is developed,which based onthe charactristic areas of the u nit step response of the procase,By this method,it is easy forauto-tuning PID controller with Ziegler-Nichols like tuning formula. Results of digital simulationshow that this method is effective.