The heat capacity ofa‐Si thin film prepared by electron beam evaporation method was measured from 360 to 820 K by a differential scanning calorimeter. For the as‐prepareda‐Si specimen, two novel irreversible endothermic processes and one irreversible broad exothermic reaction were found. The origins of the endothermic reactions were not known. It is suggested that they may be caused by a change in the number and distribution of voids that occurs at approximately 465 K and the creation of dangling bonds at the higher temperature regime (≳620 K). The exothermic reaction is attributed to heat release during structural relaxation. When measuring the heat capacity ofa‐Si,Ca‐Sip, these irreversible reactions were first eliminated by annealing the specimens at high temperatures. The heat capacity of crystalline Si,Cc‐Sip, was also measured and the difference, &Dgr;CSip=Ca‐Sip−Cc‐Sip, was used to evaluate the thermodynamic melting temperature of thea‐Si,Tal, which is determined to be 1400 K.