Amorphous phase and crystalline phase formation, and phase transition by solid-state reaction were observed in Zr/Si multilayer thin films by differential scanning calorimetry and X-ray diffraction. The result was compared with those expected by the effective driving force and effective heat of formation models. An amorphous reaction occurred in Zr/Si multilayer thin films and it was consistent with that predicted by the effective driving force model. The first crystalline phase formed by solid-state reaction was found to be ZrSi in the Zr/Si system. According to the effective heat of formation model, however, ZrSi, was expected to be formed first. The difference in first crystalline phase between the experimental result and the predicted one is discussed. ZrSi2 was formed after the formation of ZrSi, regardless of the atomic concentration ratios of Zr/Si multilayer thin films. The rate-controlling step for the formation of ZrSi was a nucleation of ZrSi and the activation energies for the formation of ZrSi and ZrSi2 were 1.64 +/- 0.19 eV and 2.28 +/- 0.36 eV, respectively.