Abstract: The study was to explore children's development of judgment of expected value in Chinese primary school Subjects from grade 1, grade 3 , grade 6 and college students participated in the study They were asked to give judgment of expected value in 5 kinds of tasks The results showed: (1) Even 7 year old children could make probability inference and have correct judgment of expected value in simple tasks; (2) Children's judgment of expected value developed with age, they consider both probability and value dimensions, and gradually find their relation of multiplication, but their level of use of multiplication rule is lower than adults (3) Under the condition that expected value is equal, children pay more attention to probability than to value

Key words: children, probability, expected value