“Expectancy” has a variety of meanings in psychology, but in this study, it means one's preparatory state before really taking action. To form such “expectancy”, stimuli must be regularly presented, and Ss must be aware of such regularity. The purpose of this study was to investigate in what condition and where, provided stimuli were regularly presented, preschoolers (4-5-6-year-old) could form expectancy using a simple reaction time experiment paradigm. The main results were as follows: 1)4 and 5 year-old children could form expectancy, only when they “interpreted” the stimulus regularity into their own sensori-motor actions, i.e., when they reacted to the cued stimulus during experiments. 2)In contrast to their expectancy, the 6 year-old children did not necessarily need such “interpretation”.