The impact of dispersed generators on reliability indices in distribution networks is analyzed. The reliability indices of a typical distribution network adopting reclosing， automatic backup power switching， anti-islanding and intentional islanding are calculated， respectively. The evaluation model and the SAIFI-Island curve for evaluating the reliability are also derived. The results show that the dispersed generators often have a negative impact on the reliability indices through the reclosing failure and other improper operation strategies. The anti-islanding relay can rule out these negative impacts， and the proper intentional islanding coordinated with auto reclosing can make an improvement on system reliability.