One critical difficulty in implementing Merton’s (1974) credit risk model is that the underlying asset value cannot be directly observed. The model requires the unobserved asset value and the unknown volatility parameter as inputs. The estimation problem is further complicated by the fact that typical data samples are for the survived firms. This paper applies the maximum likelihood principle to develop an estimation proce- dure and study its properties. The maximum likelihood estimator for the mean and volatility parameters, asset value, credit spread and default probability are derived for Merton’s model. A Monte Carlo study is conducted to examine the performance of this maximum likelihood method. An application to real data is also presented.