The slant rectangular ribs (SRRs) arranged in a microchannel can enhance heat transfer efficiency due to the flow disturbation, the interruption of boundary layer and increase of heat transfer area. In order to fully understand the mechanism of heat transfer enhancement, the flow and heat transfer in the microchannel with single- and double-row slant rectangular ribs are numerically simulated at Reynolds number ranging from 62.5 to 625. Numerical results show us that, great difference of flow pattern between microchannel heat sinks (MCHS) with single- and double-row slant rectangular ribs exists, and enormous influence of parameters of SRRs on heat transfer presents in various conditions. The flow disturbation of SRRs is the crucial factor to enhance heat transfer. Thereafter, the parameters such as attack angle, height, length, width and pitch of SRRs are evaluated by the average Nusselts number with identical pump power. Solid foundation for design of MCHS with SRRs can be provided.