Personality traits are stable, internal characteristics that enable people to summarize, explain, and predict behavior. People often make inference about behavior based on personality traits. It is suggested that trait inference may be made at two different levels: behavioral and psychological. During the past three decades, researchers have been interested in children's understanding of traits, and a large amount of research has been conducted to investigate the development of trait inference. Before the 1990s, the majority of studies focused on children's trait inference at the behavioral level by using behavior predictions. After the 1990s, researchers studied the psychological cause of behavioral regularity by examining children's theory of mind and internal state predictions. However, less attention has been paid to direct comparisons of the two levels of children's trait inference and the rules they use to make behavioral and psychological predictions. The aim of the present study was to investigate the development of trait inference at the two levels and the use of different rules that children used in trait inference.The participants in the study included 160 children at 4 to 12 years and 32 undergraduate students. Trait inference was measured through behavioral and emotional predictions, and the information was collected about the rules children used to make trait inference.The major results are as follows. First, children at least from age 4 were capable of making trait inference at the behavioral level, and from age 5 capable of making trait inference at the psychological level. Chidren's trait inferences at both levels matured at 10 years. Second, trait inference at the psychological level fell behind trait inference at the behavioral level at ages 4 and 5, and there were no significant differences between the two types of inferences at age 7. Third, children used a conceptual similarity rule rather than a simple situation-matching rule to predict behavior. Finally, children at 4 years did not employ a simple valence rule to make cross-trait category behavioral predictions, whereas children at 5, 7, 10 and 12-years and adults showed evidences of using this rule in a clearly discriminating way.These findings indicate the developmental trend of trait inference at the behavioral and psychological levels in Chinese children. The study also provides some new information about the strategies that children might use to make behavioral predictions.