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By investigating the nature of the first-order magnetostructural phase transition, we point out the origin of large overestimation of magnetic entropy changes calculated by directly applying the Maxwell relation. Furthermore, if the mass variations in ferromagnetic and paramagnetic phases on temperature are taken into consideration in two-phase region, taking MnAsC(x), (Mn,Al)As, and Mn(0.994)Fe(0.004)As compounds as example, based on the modified Maxwell relation, the large overestimation and discrepancy of magnetic entropy changes disappear. The magnetic entropy changes are equivalent with the values by Clausius-Claperyon equation, indicating the validity of this modified approach. (C) 2010 American Institute of Physics. [doi:10.1063/1.3446843]