The analysis of the existing way and amount of profile modification of cycloidal gears at present showed that the equidistance and movedistance modification is in substance really to have the profile swelled up,and the rational modification way and amount is a swelling correction within the range of zero to some limit. This paper presents a new modifying way which is optimum compound modification at separate parts. The modified profile has the most simultaneously transmission teeth,even side clearance and the longest wear-life to forming crush dressing roll and is also easyly to NC machining.