In order to investigate the influence of protein on the phase transitions of amorphous structure of sugar, we carried out differential scanning calorimetry (DSC) analysis of freeze-dried sucrose containing various contents of water and bovine serum albumin (BSA). Glass transition temperature Tg, crystallization temperature Tcr, and melting temperature Tm were measured. Tg, Tcr, and Tm decreased with increasing water activity. Addition of BSA raised Tg except the case that water activity was low (up to about 0.1). Tcr increased linearly with BSA content, and Tm did not depend on BSA content. The increase in Tg and Tcr with BSA suggests that protein contributes to stabilizing amorphous structure of sugar.