The initially formed defects in the mold of continuous casting are likely to be expanded and propagated in the subsequent processing. Based on the principle of material mechanics, a mathematic model is proposed to describe the development of the solidifying shell at meniscus for continuous casting steel slab of low-carbon ([C] ¼ 0.04%) that with the crosssection of 1200 mm  200 mm. The influence of mechanical stress induced by the mold oscillation on the growth state of meniscus shell and the primary dendrites is evaluated in combination with a non-sinusoidal oscillation mode of mold. The results suggest that the stress caused by the shell bending tends to exceed the critical fracture strength of initial solidifying shell at high temperature. The initial surface crack is formed easily when the upward oscillation velocity of mold exceeds 1.6 m min À1 and is located at 1.9-5.6 mm below meniscus. The subsurface crack is formed when the downward velocity oversteps 6 m min À1 during the negative strip time and is produced in the distance from 3.1 to 4.3 mm below meniscus. The growth direction of primary dendrite changed regularly along with the mold oscillation.