The Nb-16Si-xTi-yZr-zHf (x=18, 22; y=0, 4; z=0, 4; at.%) alloys were prepared by arc melting to investigate the influence of elements Ti, Zr and Hf additions on the phase constitution, microstructure, fracture toughness and crack propagation behavior. The results show that the single addition of 4 at.% Zr promotes the eutectoid reaction of (Nb,X)₃Si phase to Nb solid solution (Nbss)/γ-(Nb,X)₅Si₃ eutectic, and simultaneous addition of Ti, Zr and Hf further promotes the eutectoid reaction. The crack trends to propagate in (Nb,X)₃Si phase, and the crack deflects when the crack meets Nbss. The fine Nbss/γ-(Nb,X)₅Si₃ eutectic and Nbss/γ-(Nb,X)₅Si₃ lamellar structures can induce the crack bridging and branching, and hinder the crack growth. Nb-16Si-22Ti-4Zr-4Hf alloy with the highest content of alloying elements exhibits the highest room-temperature fracture toughness (11.62 MPa·m^1/2), which is 87.7% higher than that of the Nb-16Si-18Ti alloy. The performance improvement is mainly attributed to the presence of lamellar eutectic structure.