AbstractThis study examined developmental change in young children's moral judgments of commission and omission related to mental states, especially knowledge or ignorance. 4–5 and 5‐ to 6‐year‐olds (n=67) made moral judgments about the tasks related to the understanding of knowledge or ignorance. The tasks were also composed of two types of acts: commission or omission. The results showed that the both age groups understood knowledge and ignorance, but that the older group made moral judgments based on this understanding more similar to adults compared to the younger group. There was not an age difference concerning whether the acts were of commission or omission. These findings indicate that there is no difference for young children in the difficulty in moral judgments of acts of commission and omission related to mental states, whereas there is a developmental difference in using the understanding of knowledge or ignorance for making moral judgments. Copyright © 2009 John Wiley & Sons, Ltd.