A new type of optical diffuser with cascaded microlens arrays (MLAs) is proposed. The diffuser is composed of a layer of MLAs with small aperture and high numerical aperture (NA) on output side and a layer of MLAs with large aperture and low NA on incident side. The influence of the parameters (such as sag height, aperture etc.) of cascaded MLAs on the diffusing behavior is investigated by using of ray tracing method. The results show that the maximum luminance increases with the ratio of sag height to aperture and an additional 3%～5% optical gain can be accomplished by inserting another layer of MLAs with a height-aperture ratio of 0.01～0.15 on incident side. Experimental results are also demonstrated and the optical gain is measured as 40.06%.