Nb-20Ti-16Si-3Al-3Cr-2Hf alloy was prepared by non-consumable electrode arc-melting, alloyed with different content of rare earth elements La, Sm and Tb. Microstructures and phase composition were analyzed. The results show that (Nb,Ti) and Nb₅Si₃ are the main phases. Diverse microstructures in different position of the ingots and coarse phase concentration are observed; eutectic grains and lath-like grains with large Nb₅Si₃ phase as the core widely exist in these ingots; the eutectic grain contains lamellar structure internal and coarse "teeth like" structure external; the Nb₅Si₃ phase in the center of lath-like grains persists straight interfaces and regular edges, which is surrounded by fine niobium silicide network and coarse dendritic niobium solid solution (Nb_ss) phase. Multiple linear regression is used to analyze the relationship of REEs' content and room temperature fracture toughness of the alloy,result indicates that room temperature fracture toughness of these ingots is 11-15 MPa·m^1/2. The regression equation of REEs' content (Sm, La, Tb) to room temperature fracture toughness K_q is K_q=10.344+6.896La+2.993Sm.