A shift control system for an automatic power transmission is capable of optimizing operation timing of a timing valve and whereby eliminate a shift-shock and occurrence of racing. The system includes an input device including a detector for detecting a vehicle speed and a load detector for detecting an engine load, a switching device for switching states of the friction elements for establishing the transmission speed ratios, a switching controller for selecting a transmission speed ratio among the speed ratios on the basis of the inputs from the input device and issuing a shifting demand for the switching device, a measuring device for measuring an elapsed time from a timing of recognition of the shifting demand from the switching controller, a timing adjusting device for adjusting response of hydraulic pressure supply for the friction elements upon down-shifting, a weighting and summing derive for deriving weighted sum of inputs including at least the vehicle speed, the elapsed time from the timing of recognition of the shifting demand and the engine load, upon down-shifting, and a timing control device for outputting a control signal to operate the timing adjusting devicee to perform a predetermined operation when the weighted sum derived by the weighting and summing device exceeds a predetermined threshold value.