Fe-Ni based alloys are used in high temperature for a long time, so the microstructure stability at high temperature is one of the important indexes of the alloys. The microstructure and properties evolution of a new Fe-Ni based superalloy 21Cr-32Fe-41Ni at 750 ℃ for different aging time were investigated by scanning electron microscopy ( SEM ), transmission electron microscopy ( TEM ), electron probe microanalysis ( EPMA ) and chemical phase analysis. The results show that the precipitates in the alloy after long-term aging were mainly γ′, σ, α-Cr, MC phases. The weight percentage and size of the dispersed spherical γ′ in the matrix increased rapidly within 500 h aging. With the increase of aging time from 500 h to 2000 h, the weight percentage and size increase rate of γ′ gradually decreased. During the long-term aging process, the σ phases were distributed along the grain boundaries in block or strip shape, and within grains in needle or strip shape. The distribution of α-Cr phases in the grains was the same as that of σ phases. With the increase of aging time, the number of σ and α-Cr phases increased and they gradually coarsened. The σ phases with discontinuous distribution along the grain were gradually connected, and there was a tendency to develop into a network distribution. With the increase of aging time to 2000 h, the strengths of the alloy increased first and then decreased. They reached the peak after aging for 500 h. The hardness kept increasing.