The variation of microstructure evolution, phase composition, solidification path and tensile properties of high Nb-TiAl alloys alloyed with β stable elements Cr or/and Mn were studied in this work. The results showed that volume fraction of B2 phase gradually increased while α2 phase gradually decreased in as-cast microstructures of Ti-45Al-8Nb-0.4B (atom fraction, %, similarly hereinafter) alloyed by 2Cr, 2Mn and 1Cr1Mn. 1Cr1Mn alloy turned into the novel β-γ high Nb-TiAl alloy, which was composed of γ and B2 two phases rather than γ, α2 and B2 three phases. The solidification path changed from L→β→β+α→α+γ+β→α2+γ+B2 to L→β→β+γ→B2+γ. The co-addition of Cr and Mn had more obvious β stable ability. The room temperature tensile results showed that with the increase of B2 phase, the strength and elongation both decreased. While the elongation decreased firstly and increased subsequently at 900℃. This phenomenon showed that when the β phase content reach a certain level, which was 14.4 % in this study, β phase was advantageous to deformation at high temperature.