In order to investigate the impacts of large scale DGs on the power system transient and voltage stability, the dynamic models of the power system and DGs in a 12-bus test system are modeled in detail by PSCAD/EMTDC software. And then through simulation and theory analysis, the impact of the DGs with different penetration level on the maximum rotor speed deviation and the rotor speed deviation frequency of the main synchronous generators are studied, and the impact of the DGs with different control models on the power system bus voltages, before a fault (steady state), during a fault and after a fault is analyzed. The research results show that the inverter interfaced DGs have a more negative impact on the power system transient stability with the penetration level increasing, but the voltage controlled inverter interfaced DGs significantly improve the voltage in the power system buses after fault clearances.