Taking Fe-0.15wt%C-0.8wt%Mn steel as the research object, in situ observations of the δ phase growth process during solidification were conducted by using a high-temperature confocal scanning laser microscope (HTCSLM). The effects of the solid/liquid (S/L) interface'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 during solidification. The results indicate the growth rate of the concave-shaped S/L interface is larger than that of the convex-shaped S/L interface 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℃/s.