In solidification of low-alloy steels, micro-and macrosegregations are greatly affected by peritectic reaction which determines the redistribution of solute elements during solidification and effects the susceptibility of internal crackings in continuous slab casting. However, little is known about the effect of alloying elements on the peritectic temperature. In this study, therefore, the effect of alloying elements on the peritectic temperature in low-alloy steels was studied using a differential thermal analysis and a new method was developed to construct quasi-binary phase diagrams for multi-component systems. It was found that Mn and Ni increase the peritectic temperature and Si, P, S, Cr, Mo, and V decrease it. This tendency was well explained in terms of α/γ phase stability parameter. Moreover, the accuracy of estimating solidus temperature was improved by taking the change in peritectic temperature into account.