Ni3Si compound is one of the excellent high temperature structural materials, because it possesses the attributes of high melting point, high strength, low density, excellent oxidation resistance at elevated temperatures, and magnificent corrosion resistance in acid environments, particularly sulfuric acid solutions, while the application of this compound is limited due to poor ductility at ambient temperatures and lack of fabricability at high temperatures. The incorporation of a ductile phase into the intermetallic materials has become an attractive means to modify the ductility. In this paper Ni-Ni3Si hypoeutectic in situ composites are obtained by Bridgman directional solidification technology. Microstructure of the Ni- Ni3Si hypoeutectic in situ composites are regular lamellar eutectic structure at the lower solidification rates, whereas eutectic cells or dendrites can be found with the increase of the solidification rate, due to the increase of the composition undercooling. Moreover, the directional solidification mechanism was investigated as well.