The effect of hourly load change on the treatment characteristics in an anaerobic-aerobic activated sludge process was compared for two types of variation. When the variation of wastewater concentration or flow rate was applied to the treatment unit according to normal distribution function like effluent pattern in actual life, the treatment system gave relatively stable and successful removal of nitrogen and phosphorus compounds. When the mass flow rate of nitrogen applied to the treatment unit was equal, the performance of the present process was almost same regardless of load variation mode. The simulation of treatment behavior was carried out successfully by using the kinetic equations for sludge floc and the reactor models which considered the treatment unit as a completely mixed tank.