TEM, EDS and STEM were used to investigate distribution of Mo element in new CrMo steel ADF1 which is applied to 1300MPa high strength bolts with superior delayed fracture resistance. Research results reveal that Mo easily concentrates at grain boundaries. According to refined EDS point and mapping analysis, the region of Mo segregation at grain boundary is about several nanometers wide, and distribution of Mo along the grain boundary is not uniform. The average content of Mo at grain boundary is about double of that in grain area. Concentration of Mo can increase the bonding force of grain boundary, and this is of advantage to the improvement of strength and delayed fracture resistance of the new CrMo steel.