The voltage instability phenomena may occur in distribution networks. The decline of voltage stability level will restrict the increase of load served by distribution companies. Distributed generation (DG) is increasingly drawing great attention of people. The development of DGs will bring new chances to traditional power systems. DGs connected to distribution networks are potential to improve the system voltage stability. In this paper, the familiar interfaces between DGs and distribution networks are introduced. The operation modes and control characteristics of typical DGs, including asynchronous generators, synchronous generators with constant values of excitation voltage, and fuel cells, are analyzed. Based on the models of DGs proposed in power flow calculations, a new method for constructing a sensitivity matrix is presented, which is available to calculate the power flow of distribution networks with multiple types of DGs. Using this method presented here, the impacts of DGs on the system voltage stability are studied. These impacts are quantified by a voltage stability index introduced in this paper. Some important conclusions are obtained. A 90-bus system is analyzed to give an illustration of the feasibility of the proposed method