Accurate heat capacities of the single-molecule magnet [Mn12O12(O2CEt)(16)(H2O)(3)] were measured from 0.3 to 311 K. by adiabatic calorimetry without an external magnetic field. Heat-capacity anomalies were separated by assuming several contributions including lattice vibration, magnetic anisotropy, and hyperfine splitting. Among them, a tiny thermal anomaly between I and 2 K is attributable to the presence of Jahn-Teller isomers. The heat capacities of the polycrystalline sample were also measured with applied magnetic fields from 0 to 9 T in the 2-20 K temperature region by the relaxation method. With an applied magnetic field of up to 2 T, a steplike heat-capacity anomaly was observed around the blocking temperature T-B approximate to 3.5 K. The magnitude of the anomaly reached a maximum at 0.7 T. With a further increase in the magnetic field. the step was decreasing, and finally it disappeared above 3 T. The step at T-B under 0.7 T can be roughly accounted for by assuming that a conversion between the up-spin and down-spin states is allowed above T-B by phonon-assisted quantum tunneling, while it is less effective below T-B. Excess heat capacity under a magnetic field revealed a large heat-capacity hump around 14 K and 2 T. which would be attributed to thermal excitation from the S = 9 ground state to the spin manifold with different S values, where S is the total spin quantum number.