In this work, the nanocrystalline SmCo8.9Si0.9 alloy with a high stability was first prepared and then a systematic study was carried out on the phase transformation and corresponding magnetic properties of the metastable SmCo8.9Si0.9 phase.The results revealed that the stability of the metastable upersaturated solid solution SmCo9.8 could be greatly improved for the addition of Si. With the increase of the heat-treatment temperature, the nanocrystalline SmCo8.9Si0.9 alloy transformed from the SmCo9.8(H) phase to the Co(FCC) phase and the coercivity was correspondingly improved. The mechanism was that the precipitated fine Co phase enhanced the pinning effect. With a further increase in temperature, the Sm2Co17(H) phase transformed into Sm2Co17(R) accompanied with an evident grain growth and an inhomogeneous grain size distribution, leading to the decrease in the magnetic properties.