Abstract
Maxwell relation and mean-field approximation are chosen to estimate the total entropy change and the magnetic entropy change, respectively. The experimental results are exactly consonant with the theoretical results. Consequently, the magnetic entropy changes make up the majority of the total entropy change, and the lattice entropy changes are small. The giant magnetocaloric effect in MnAs is clarified. According to thermodynamics discussion, the modulus of compression and the stress at TC decrease dramatically. Therefore, the magneto-structural phase transition in MnAs is a kind of soft mode.