The purpose of this study is to review developmental studies of the conception of time. The conception of time may be one of the base of physical world. Many researchers have experimentally examined the development of conception and knowledge structure of time after Piaget (1946 / 1969). One of the most popular experimental situations that Piaget originated is as follows : Children are shown two toy cars that travel in the same direction on parallel tracks and are asked whether the cars ran for the same duration, and if not, which car ran for the longer duration. The result of these studies showed that it was very difficult for even 9 - 13 year old children to solve logically some of this kind of task. The difficulty seemed to be because of children's inability to use appropriately the knowledge of time. The knowledge of time involves two structures : one in which time is defined by starting and ending times ; the other in which time is defined by the relation between speed and distance. It was suggested that only the latter knowledge structure was always activated when children compared the duration of two cars in the tasks.