Abstract:The rationality of a bit top structure concerns the bit performance.There-fore,the traditional four edge symmetry structure has been changed to a three edge salidstructure which has a bigger centre plane face and double surrounding inclined faces,thatis,the drum shape structure.This paper describes the top structute of Mode C down-the-hole butten bit,including the top shape,fragments blowdown space,butten shape and but-ton distribution,etc.