In this study, we investigated the relationship between amorphous structure and physical heat resistance in polylactic acid (PLA) injection moldings. When the amorphous PLA moldings underwent heat treatment at a temperature below the glass transition temperature (Tg), the heat distortion temperature (HDT) increased without cold crystallization. Also, we clarified that there is an optimum heat treatment temperature in terms of the increase in HDT. Comparing heat treatment temperature dependence of the increase in HDT and enthalpy relaxation, we concluded that the increase in HDT of the amorphous PLA moldings due to the heat treatment below Tg is related to enthalpy relaxation. Also, we investigated the effect of heat treatment on HDT in the PLA injection moldings after cold crystallization. The heat treatment also relieved excess enthalpy in the crystalline PLA moldings. Furthermore, the segmental motion associated with α - relaxation in the crystalline PLA moldings was suppressed to the same extent as in the amorphous PLA moldings, but the degree of increase in HDT due to the heat treatment was much smaller than in the case of amorphous PLA moldings. On the other hand, comparison of the deflection curves before and after the heat treatment revealed that the temperature at which the amount of distortion in HDT begins to increase was greatly raised by the heat treatment in the crystalline PLA moldings.