This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures (900–1200 °C) and strain rates (0.001–100s−1) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125°-1200 °C and strain rate range of 0.001–0.1 s−1. High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.