The concept of competence hypothesis identified by Heath and Tversky (1991) proposes that ambiguity aversion is affected by the subjective competence level of participants. When people feel knowledgeable or skillful in a field, they would rather bet on their own judgment even with uncertainty. However, when participants do not feel knowledgeable or skillful, they prefer betting on the unambiguous chance event. Therefore, the bias of ambiguity aversion is conditional on the subjective competence level of the participants. Our study selects managers of open and domestic stock type mutual funds to be study objects. There are 12 months during the study period, starting from April 2004 and ending in March 2005. In the first part of this paper, we investigate whether the competence effect influences professional managers¡¦ behaviors. We explore the relation between manager competence and mutual fund holding bias by conducting multiple regression analysis. Our study finds competence effect in the behaviors of mutual fund managers. A positive relation exists between manager competence and mutual fund holding electronic stocks bias. When manager competence is high, the mutual fund holdings in electronics stocks will be either more or less than the market. That is, weighting of holding electronics categorized stocks in mutual fund clearly deviate from the weighting of electronics categorized stocks in the market. However, when manager competence is not high, weighting of holding electronics categorized stocks in mutual fund follows that in the market. The other hypothesis proposes that manager competence is not relevant to the £] of mutual fund portfolio. Secondly, we explore the factors that affect manager competence by conducting logistic regression analysis. Among the five variables which may affect mutual fund manager competence, we find ¡§fund size¡¨ the only factor that is positively relevant to mutual fund manager competence. In other words, the larger the mutual funds, the more likely that managers will perceive themselves as skillful and knowledgeable. The other four variables are the manager¡¦s previous fund management experience, educational level, gender, and performance of mutual fund in the past year. They are not related to competence.