Glass-transition behavior of hydrated bovine serum albumin (BSA) aqueous solutions depending on hydration degree within 0.05 and 0.43 h (water-g/protein-g) was investigated according to the measurement of an enthalpy-relaxation rate deduced from the temperature drift observed by adiabatic calorimetry. The results showed that an increase in hydration degree enlarged the temperature range in which the enthalpy-relaxation effects emerged. The BSA sample in hydration degree around 0.05 h showed one T-g peak, and further hydration to 0.24 h led to up to three distinguishable T-g, and finally BSA solution of 0.43 h showed cold crystallization due to the portion of water molecules included. The multiplicity of the T-g in the well-hydrated BSA is explained as follows: At sufficiently low temperatures, similar to 50 K, the hydrated BSA molecule and the contained water molecules are completely frozen. Then, heating induces mobility of some of the water molecules initially, and further causes the sequential unfreezing of the polar side chains of BSA molecules, coupling with the directly hydrating water molecules, and, in turn, of the main chains of the BSA molecule. This interpretation may integrate the unique insights of glass transition behavior of hydrated proteins into a consensus.