We investigated the effect of deformation temperature and gain size on the tensile behaviors of a new medium Mn-TRIP steel processed by batch and continuous annealing. It was found that the tensile strength decreased when the testing temperature increased from −20 to 300 °C. The total elongation firstly increased, reaching the maximum at 25–100 °C, and then decreased when the temperature was higher than 100 °C. The results can be explained by (i) the influence of temperature on chemical driving force for martensitic transformation and (ii) the grain size effect on the stability of austenite. To achieve optimal mechanical properties, the stability of austenite should be tailored so that the transformation-induced plasticity effect occurs under continuous deformation. Also, ferrite should have appropriate grain sizes so that work hardening of ferrite can coordinate the deformation of austenite.