High Nb-TiAl alloys, which being regarded as a new generation TiAl alloy, had attracted more and more attention for their higher operating temperature and better oxidation resistance than conventional TiAl alloys. It was found that silicide particles in high Nb-TiAl alloys were Nb₅Si₃ rather than Ti₅Si₃ precipitated in TiAl alloys. In this work, the effect of Nb₅Si₃ phase on the microstructure and room-temperature tensile properties of high Nb-TiAl alloy was studied. The experimental results showed that the precipitation temperature of silicide was between 1000~1200 ℃. Precipitates located in the colony boundary, b(B2) segregation and between g/a₂ lamella. The tensile properties of as-cast alloy with Si addition increased. Because the formation of Nb₅Si₃ precipitates resulted in the reduction of Nb content, which was one of b(B2) phase stable elements. Therefore, the volume fraction of b(B2) phase obviously decreased due to Si addition. However, after heat treatments, the tensile properties of Si containing high Nb-TiAl alloy gradually reduced with the increasing of heat treatment temperature. Silicide particles which precipitated along lamella leaded to generation and propagation of cracks. Moreover, silicide particles further precipitated due to tensile stress which increased the rate of crack propagation. Si addition leaded to g phase area expanded. g single-phase region formed between 1280~1300 ℃. Silicide precipitated in colony boundary resulted in bulk g+b(B2) phases, which weaken the grain boundaries. While silicide precipitated in lamella leaded to formation of secondary g lath which split the initial lamella microstructure.