Rapid solidification of Co-Si alloys was investigated by using vacuum suction casting in this study. Different microstructures and intermediate phases were obtained. Eutectic εCo phase and eutectoid εCo + αCo2Si structures were obtained in the first eutectic Co76.9Si23.1 alloy. The microstructures of hypereutectic Co70Si30 alloys were composed of primary αCo2Si phase and interdendritic lamellar eutectoid εCo + αCo2Si. While for hypoeutectic Co63Si37 alloy at the second eutectic point, CoSi dendrites were the primary phase, and αCo2Si+CoSi eutectoid structures can be seen at the interdendritic region. Especially, a metastable CoSi2 phase was found in Co63Si37 alloy. This indicates that eutectoid decomposition βCo2Si→ CoSi+αCo2Si is restrained in eutectic Co60.3Si39.7 alloy. For rapid solidified Co55Si45 and Co52Si48 alloys, αCo2Si+CoSi eutectoid structures were not observed, while metastable CoSi2 were obtained. The higher hardness achieved in Co-Si alloys at the second eutectic point, for the reason of the higher volume fractions of compound phases.