This work aims at studying the change in constituent phases of High Manganese High Aluminum content steel through isothermal ageing and their effect on the plastic behavior of the produced steels. Optical and scanning electron microscope were employed for observing the significant change in microstructure at different heat treatment regime. XRD was applied to detect the major phases after isothermal process. Three samples with different phase consitituents were subjected to compression test. The results refer to the decomposition of γ-austenite into β-phase (B2, DO3), β-Mn, K-carbide is widely changed as a result of isothermal ageing process. The plastic behavior and strain-hardening property are improved linearly with k-carbide fraction. A bit enhancement of hardness has been observed with increasing k-carbide fraction after isothermal ageing process.