This paper defined instream ecological flow, and minimum, maximum and optimal instream ecological flow respectively, proposed a new monthly frequency method for instream ecological flow computation, i.e. the same equal-frequency value being assigned for different months, and analyzed its advantages, disadvantages and applicability. The paper selected the Yihe River in Luoyang city, China as case study site, employed the proposed method to compute the instream ecological flow processes at two typical hydrological stations, and evaluated the computed instream ecological flow by Tennant Method. The results revealed that monthly optimal instream ecological flow rates computed by the proposed method are higher than those by the traditional monthly frequency method, i.e. different frequency values being assigned for different months. The recommended optimal instream ecological flow requests more water for environmental use and more benefits for river health and the habitat conservation in river floodplain and riparian corridors.