A unique dataset of intra-month round-trip transactions of high-turnover-rate mutual funds provides this research a vehicle to uncover the evidence of a well-known behavioral anomaly-disposition effect, i.e. loss aversion, among mutual fund managers in Taiwan. This study finds on average in a 10-year period from 1999 to 2008 these funds hold losing securities significantly longer than profiting ones. The data of round-trip transactions is free from the issue of portfolio-rebalancing. It also shows that this phenomenon is not justified due to the fact that the holding period is significantly inversely related to the negative returns and positively related to the positive returns.