To study the feature of default clustering in credit risk management, this paper evaluates joint default probabilities and default correation for multiple firms through the classical de-fault barrier structure form model. Under the commonly used factor model, by making use of a developed renewal theory, we provide a closed form approximation for the joint default probability, default correlation and expected default time, both for a specific firm as well as the first to default among a group of firms. Moreover, based on the approximated default corre-lation and expected default time, we propose a multi-name Distance-to-Default, which can be used to explain the feature of corporate default clustering: contagion and co-movement. Simu-lation studies for evaluating a multi-name Distance-to-Default are given to illustrate our model.