Abstract: A research is made of the phase and structure of LaFe_11.6Si_1.4 alloys undergoing a heat treatment at different temperatures, and to explore the mechanism of the phase transition. The LaFe_11.6Si_1.4 cast alloys undergoes the peritectoid reactions and peritectic reaction under high temperature heat treatment. At the temperatures of less than about 1423K, the cast alloy undergoes LaFe_11.6Si_1.4 Fe(Si)+ LaFeSi → 1:13 Solid Solid peritectoid reaction; at the temperatures of 1423～1593K, it undergoes Fe(Si)+ LaFeSi → 1: 13 solid-liquid peritectic reaction; while in temperatures of 1593～1739K, 1:13 has formed a molten phase, Fe alloys are found in a solid/liquid two-phase state. Based on factors of phase transition thermodynamics and kinetic phase transition, cast alloy at 1523K possesses a maximum peritectic reaction driving force, after 5 hours of heat treatment of LaFe_11.6Si_1.4 alloy, the 1:13 alloy is of the highest phase content.