Abstract: The factors that cause longitudinal surface cracks formation on the Pangang's continuous cast 09SiVL steel slabs are analysed. The results show that carbon content in molten steel is the decisive factor of longitudinal surface cracks formation on the slabs and that effect of viscosity of casting powder on Iongitudinal surface cracks formation on the slabs is obvious. ln order to remove the longitudinal surface cracks. the carbon content in molten steel must be controlled in the range of 0.08%~0.10%. When the carbon content in molten steelis in the range of 0.11%~0.13%, the casting powder of medium viscosity is used so as to decrease the longitudinal surface cracks of continuous cast slab.