With a large scale of distributed generations（DGs） being installed and operated in distribution network， variety of power sources and complexity of their operation will greatly effects the reliability of distribution network. Different DGs'models and various operation situations have been studied， in which a network model called zone-nodes including DGs is introduced. The calculation model of reliability is build based on different failure types affected by zone-nodes， and the principle of island operations is definite. Then， on the above basis， a Monte Carlo simulation method is applied to solve this model. An example demonstrates the validity and practicability of the model and simulation method. The result of test example indicates that DGs can improve the reliability of distribution network， but also have some adverse impacts on it. The traditional reliability indices can not perfectly evaluate advantages or disadvantages of DGs'impact， it is recommended to standardize the evaluation system of the impacts of DGs on distributed system.