The first purpose of this study was to examine the difference of hostile attribution tendency between aggressive and non-aggressive preschoolers. The second purpose was to examine the effects of mood manipulation on hostile attribution in preschoolers. Aggressive and non-aggressive preschoolers were administered tests of attributional tendencies in hypothetical ambiguous conflict situations. After the test, the subjects had their mood manipulated by winning or losing computer games. The main results showed that aggressive preschoolers displayed more hostile attributional biases than non-aggressive preschoolers, and that both aggressive and non-aggressive preschoolers exacerbated hostile attributional biases under negative mood manipulated conditions.