Abstract: To improve the sensitivity of current differential protection for distribution networks with inverter interfaced distributed generation (IIDG), this paper analyzes the differences in fault current waveforms between IIDG and synchronous generators. There are huge differences in current waveforms on both sides when in-area faults occur on the line, and relatively small differences in current waveforms on both sides when out-of-area faults occur on the line. A longitudinal protection scheme is proposed based on the similarity coefficient of current waveforms on both sides of the line. The similarity coefficient is built by utilizing the differences of cosine similarity between the two sides of the line currents, so as to realize the adjustment of the protection. The simulation results show the protection scheme correctly identifies in-area and out-of-area faults, deals well with transition resistance and synchronization error. Moreover, it effectively improves the protection sensitivity compared with the traditional current differential protection.