Abstract: A lock-up clutch control system for an automatic power transmission includes a torque converter mode, a full lock-up mode and a clip lock-up mode for controlling operation of a lock-up clutch which is disposed between a pump impeller and a turbine runner of a torque converter. The slip lock-up mode is a transitional mode between the torque converter mode and the full lock-up mode and connects the pump impeller and the turbine runner such that a relative rotation therebetween is allowed. In the system, one of the above-noted three modes is selected based on preselected vehicular operating condition indicative parameters, such as, an engine load and a vehicle speed. When a rapid depression of an accelerator pedal by a driver is detected after the slip lock-up mode is selected, the torque converter mode is executed in place of the slip lock-up mode until the rapid depression of the accelerator pedal is not detected.