Abstract
This study investigates the motivation of asset substitution problem. I find the firm-specific risk is significantly positive related with the firm's overinvestment level. My finding confirms that the asset substitution problem is motive from the firm-specific risk, because the call value that can be viewed as the value of firm's equity position is increasing when the risk of the underlying asset increases. Moreover, I use option implied bankruptcy probability to measure bankruptcy risk and find the asset substitution problem will be enhanced by high firm-specific risk especially at those high bankruptcy risk firms.