Undercooling solidification experiments were carried out to analyze the precipitation and evolution behavior of different phases of Co-Ru peritectic alloy. The maximum undercooling of this alloy up to 217 K (0.12TL) was obtained by applying glass fluxing combined with cyclic superheating method. According to the calculation results of the classical nucleation theory, the nucleation of stable phase (ξ) is favored in the whole temperature range. Results show that typical peritectic microstructures are observed if the initial undercooling (ΔT) was lower than critical value (ΔT* ≈ 180 K). In this condition, the precipitation of peritectic phase (α) depends on the different morphologies of ξ phase. When ΔT >ΔT*, α phase precipitates from the undercooled melt directly and grows competitively with ξ phase, although ξ phase is still the primary one in the undercooled melt. With increasing ΔT, the relative content of peritectic phase goes through a course of increasing, then decreasing and finally increasing again