This paper designed the flow and method of custom RC corners, customized a new RC corner, and estimated the coverage of the custom corner. The result shows that the coverage of custom RC corner to the other corners can reach up to 99%. Finally, we improved the traditional MMMC analysis flow with the custom RC corner. The result of application case in engineering shows that the runtime of tools reduced greatly at the cost of buffer count and buffer area. The buffer count increased by 22.07% and the buffer area increased by 21.65%, whereas the runtime of tools decreased by 84% after the timing was closuring.