For the multi-gray shades image in the passive matrix type STN-LCDs, the method which combined space modulated patterns and frame rate control(FRC)with switched some frames has been used. Although bi-level display elements are used this method makes it possible to display a multi-gray shades image without resolution-loss for still-image, while the method has a fault to perceive interference of flicker at low frame rate, as is wellknown. However the measured quantitative data for flicker and frame frequency have hardly be published. With regard to frame rate control method which some typical space modulated patterns are combined, we measure critical flicker frame frequency(CFFF) with subjective experiments. This paper also discribes that the data are arranged to be shown in form of relationship with the screen luminance and the lowest frequency components included among ripples on times and also the ripples ratio. Our experimental study provides useful data for designing such kinds of display.