The purpose of this study is to examine child aggression in relation to temperament and mother’s reaction to child negative emotions. A sample of 366 children in the 5th or 6th grade in six elementary schools participated. Temperament, mother’s reaction to child negative emotions and aggression were assessed by a child-report using a set of questionnaires. The collected data were analyzed with descriptive statistics, t-tests, Pearson’s correlations and hierarchical regression analyses. The major findings are as follows. First, boys reported a higher level of oversensitive-reaction of their mothers than girls. Second, boys showed greater physical aggression than girls. Third, there were bivariate associations among temperament, mother’s reaction to child negative emotions and child aggression. That is, greater aggression was related to higher negative emotionality, lower activity as well as sociability of the child. Also, greater aggression was linked to less emotion-coaching reaction, more oversensitive reaction and emotion-minimizing reaction of mothers. Fourth, child aggression was significantly explained by gender, negative emotionality and mother’s reactions to the child negative emotions. The findings suggest the importance of assessing negative emotionality of children and educating emotion coaching behaviors of mothers for reducing child aggression.