In this study, the terahertz (THz) emission in a Co2MnSi/Pt structure was investigated. The THz emission intensity increased after annealing, and the optimized temperature was above 400 °C owing to the increase in B2 ordering. The THz emission intensity of Co2MnSi/Pt was 1.4-2.0 times larger than that of CoFe/Pt. The THz conductivity for Co2MnSi/Pt was smaller than that for CoFe/Pt. The ultrafast spin-current intensity was almost the same in both materials. A small conductivity and comparable ultrafast spin-current injection into the Pt layer are possible reasons for the efficient THz emission in Co2MnSi/Pt.