Taking Fe-0.15wt%C-0.8wt%Mn steel as the research object, in situ observations of the δ phase growth process during solidifica-tion were conducted by using a high-temperature confocal scanning laser microscope (HTCSLM). The effects of the solid/liquid (S/L) inter-face’s shape, temperature, and curvature radius on the lateral growth rate of δ phase were investigated in detail, and the relational expression among the lateral growth rate of δ phase, temperature, and curvature radius of column-shaped δ phase was deduced for the carbon steel dur-ing solidification. The results indicate the growth rate of the concave-shaped S/L interface is larger than that of the convex-shaped S/L inter-face during the beginning growth of δ phase, but these two kinds of growth rates gradually approach with proceeding in solidification. The calculated lateral growth rate of δ phase is consistent with the experimental result at a cooling rate of 0.045°C/s. Taking Fe-0.15 wt% C-0.8 wt% Mn steel as the research object, in situ observations of the δ phase growth process during solidifica- tion were conducted by using a high-temperature confocal scanning laser microscope (HTC SLM). The effects of the solid / liquid (S / L) inter-face’s shape, temperature, and radius radius on the lateral growth rate of δ phase were investigated in detail, and the relational expression among the lateral growth rate of δ phase, temperature, and curvature radius of column-shaped δ phase was deduced for the carbon steel dur-ing solidification. The results indicate the growth rate of the concave-shaped S / L interface is larger than that of the convex-shaped S / L inter-face during the beginning growth of δ phase, but these two kinds of growth rates gradually approach with proceeding in solidification. The calculated lateral growth rate of δ phase is consistent with the experimental result at a cooling rate of 0.045 ° C / s.