Purposes of the present study were to reveal developmental processes of relational concepts among time, distance, and velocity. Children from 4 to 10 years old and adults were used as subjects. There seemed to be three developmental stages and two transitional periods from one stage to another: Stage I, where children could not grasp the inverse relation between time and velocity at all and were approaching awareness of the direct relations between time and distance, and distance and velocity (mainly children under 6 1/2 years of age). Stage II, where children grasped nearly correctly the three relations based on the coordination of two by two relations (mainly children between 6 1/2 and 8 1/2). Stage III, where children understood these relations based on the time-distance-velocity system.