The effects of phase transformation on the microstructures and mechanical behavior of FeNiMnCr0.75Alx (x = 0.25, 0.5 and 0.75) high-entropy alloys (HEAs) were investigated. The crystal structures of the FeNiMnCr0.75Alx HEAs transitioned from the primary FCC1 and minor BCC (x = 0.25) to primary BCC and minor FCC2 (x = 0.5 and 0.75) phases. TEM images of the as-cast FeNiMnCr0.75Alx (x = 0.25, 0.5 and 0.75) alloys showed the BCC precipitates embedded in the FCC matrix phase. After annealing, two substantial phase transitions occurred for the x = 0.25 and 0.5 alloys from the BCC phase to the σ phase and between the FCC2 phase and the FCC1 phase, effectively increasing the yield strength and sharply reducing the strain. The yield strength of the FeNiMnCr0.75Al0.25 alloy greatly increased from 450.5 MPa (as-cast) to 945.0 MPa (annealed at 900 °C), but the strain sharply decreased from above 60% (as-cast) to 24.3% (annealed at 900 °C). Meanwhile, annealing had no obvious effect on the phase transition of the FeNiMnCr0.75Al0.75 alloy. In addition, the correlation between the phase transformation BCC - σ phase and FCC2 - FCC1 was also investigated. This investigation provided a promising way to achieve phase-stable HEAs.