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A computational study is conducted to examine the influence of mechanical twins on the kinetics of martensitic transformation in TRIP-maraging steel. Simulations are performed using a non-local crystal plasticity (CP) phase transformation model describing the deformation behavior of TRIP-maraging steel. This work is inspired by the experimental observations of Wang et al. (2014), where authors reported a twinning induced stability against the phase transformation in austenite islands embedded in the martensitic matrix. Finite element (FE) analysis is used to simulate the stability of austenite against the phase transformation in the presence and absence of mechanical twins, respectively. Depending on the parent crystal orientation, macroscopic outcome of FE calculations manifested a slower/faster transformation kinetics for austenitic islands with mechanical twins as compared to islands without twins which is found to be in a good qualitative agreement to experimental observations. Models with two austenite islands of different sizes confirmed the twinning induced alteration in the transformation kinetics. Microscopic observations from the simulations demonstrated that the twin orientations generated during mechanical twinning are less or more favourable for the strain induced or stress assisted transformations. This directly indicates that the basis of the alteration in the transformation kinetics could be the strong orientation dependence of the martensitic transformation. Thus, a model based analysis was used to explain the experimental findings related to phase transformation kinetics of TRIP-maraging steels.