This study examines the effects of extension ability, and brand association on the fitness of brand extension. Real brands of watches are used to test 183 college students. The results show that transfer ability is the most important factors. High level of production difficulty, as well as more positive brand associations can also contribute to the fitness of brand extension. On the opposite of previous studies, product similarity is not related to the success of brand extension. The results can be contributed to brand managers when making the decision of brand extension. It is also suggested that the application of brand association should be further investigated in the future study.