Abstract: By presenting 2 sets of pictures representing different violating situations in which mother or child or both acted as violator(s) to 3 age groups of 4, 5, and 6 year-old, preschoolers’ understanding of social contract in the form of conditional promise was examined. The results indicated that: 1) Preschoolers’ understanding of conditional promise showed evident developmental progress, which means that most preschoolers can detect peer violation and the rate detecting mom violation and bilateral violation grows with age, among which none 4-year-olds can detect mom violation and none 5-year-olds can detect bilateral violation. 2) Preschoolers’ detection of different violating figure is different, which manifests perspective effect and priority detection for peer violation effect. 3) Most preschoolers didn’t understand conditional promise as biconditional

Key words: conditional promise, social contract, deontic reasoning, perspective effect