One of the important protection characteristics in online distributed generation is to identify the islanding operation. And in accordance with standards of distribution network, this identification must take in less Than 2 seconds. A novel islanding detection method for an inverter-based distribution generation power system is proposed in this paper. The inverter-based distribution generation power system includes a dc power source and a grid-connected DC/AC inverter. The grid-connected DC/AC inverter acts as a virtual capacitor as the frequency is slightly lower than the fundamental frequency of utility voltage. Hence, the proposed method can immediately detect the islanding operation. This method is combination of previous methods therefore its operation is more suitable. The results of simulation done by MATLAB program, shows the suitable operation of this method well. It is observed that method is highly effective in islanding detection, including different islanding and non-islanding conditions in the initial test system and standard inverter-based distribution generation power system? with wide variations in operating parameters.