Abstract: Slab, crack, fold and pit defects appeared on the surface of HPB300 steel bars. The causes of surface defects were analyzed by macroscopical analysis, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the large number of non-metallic inclusions were the main reasons of slab and crack defects, and the irregular shape of the roll groove was another cause of crack defects. The ears or the overfilled billet had presented in the front pass of the rolling process caused the fold defects. The outside metal or other material falls on the surface of the rolled piece during rolling process, and the lost meat defects was formed after the later process.