A system for controlling automatic, multi-step or continuously variable, transmission equipped with a motor vehicle utilizing fuzzy logic. In the system, fuzzy production rules have been prepared based upon an expert driver's judgments and operation made on a vehicle with a manually shifted transmission. Various operating conditions including opening degree of a throttle valve and road speed of the vehicle are detected and from the detected value, various predictions are made such as an appropriateness of vehicle response to a gearshift intended by the driver and engine output change that would occur should the transmission be shifted to a given gear. These measured or estimated value are put on a common scale named universe of discourse and membership values of the rules are successively calculated. Among the rules, one rule is finally selected and according to the gear indicated in the selected rule, the current gear is shifted up or down or is held at the present position, in case of the multi-step transmission. Similarly, the speed ratio in the continuously variable transmission is determined from the selected rule.