Cast defects of hypo-peritectic steel such as uneven growth of strand shell, crack formation and oscillation marks formation were found to occur frequently during continuous casting of steels. In industry, measures such as high-basicity casting powder, hot-top mold and reduction of mold cooling strength were usually used in the investigations, but a reasonable explanation for these measures has been lacking. In this work, solidification of 15CrMoG steel at different cooling rates were observed with an ultra high temperature confocal scanning laser microscope. The precipitation of the δ-phase was in a cellular manner when the cooling rates were 5 and 15 ℃/min, whereas it was in a dendrite manner when the cooling rate was increased to 100 ℃/min. Thermodynamic analysis of the peritectic phase nucleation showed that a concentration gradient existed at the L/δ interface during the solidication of initial δ phase which led to an increase in the Gibbs free energy barrier for the nucleation of the peritectic γ phase. As the cooling rate increased, the concentration gradient across the L/δ interface became steeper, resulting in an increase in the nucleation undercooling of the peritectic γ phase. This, in turn, decreased the temperature and increased the peritectic reaction rate. In addition, an increase in the cooling rate led to a change in the mode of peritectic transformation (δ→γ). A diffusion-controlled δ→γ transformation occurred due to the progression of planar and cellular interfaces at cooling rates of 5 and 15 ℃/min, respectively. However, a large δ→γ transformation, which was controlled by the interface process, occurred when the cooling rate was increased to 100 ℃/min. The difference in volume shrinkage of the different modes of peritectic transformation (δ→γ) led to a discussion of the control mechanism of continuous casting of hypo-peritectic steel.