Abstract: An automatic transmission control system for a transmission which is installed in a vehicle and whose torque converter is provided with a lockup mechanism; comprising a vehicle weight estimation device for estimating the weight of the vehicle; a gradient estimation device for estimating a gradient on which the vehicle is running; a standard lockup line selector for storing therein standard lockup lines which correspond respectively to the specified gear shift ratios of a speed change gear included in the transmission, and for selecting that one of the stored standard lockup lines which corresponds to the gear shift ratio in the present state of the vehicle; a lockup line corrector for correcting the selected standard lockup line in accordance with the estimated vehicle weight and the estimated gradient; and a lockup signal output device for deciding a timing of lockup "ON" or lockup "OFF" by the use of the corrected lockup line, and for delivering a lockup signal to the lockup mechanism (a lockup solenoid) at