Abstract: The  change  of structure  and  element  content in boronizing layer of steel  1.72C- 11.81Cr-0.40Mo has been analyzed  and  studied  by  using  optical  microscope,scanning  electron  microscopy  and  X-ray  analyzer. Results  showed  that the  boronizing  layer  consisted  of  white-bright  boronizing  zone(phase  FeB-phase  Fe₂B)and  black  alternating  with  white transition  zone;  there  was  a  lot  of  irregular  island  B  bearing  carbides;and  there  were  the  elements  such  as  B,C,Al,Si, Mo,Cr,Fe  in  boronizing  layer,the  B,Cr  and  Fe  content  at  front  of boride  in  boronizing  layer  was  11.48%,36.49%  and 32.23%  respectively  while  the   B,Cr   and   Fe   content   in  transition   zone  was8.07%,7.04%   and77.21%  respectively, but the Mo content was almost no change.The Cr and Mo existed in boronizing layer by carbides form,and Al and Si existed  at defect or high carbon area in boronizing layer.