Solid-state reactions in the Fe-Si system have been studied under the pressure of the hot isostatic pressure (HIP). The reaction between Fe and Si, which produces beta -FeSi2, was divided into the following steps, Fe + Si --> epsilon -FeSi (1) epsilon -FeSi + Si --> beta -FeSi2 (2) Reaction (1) was suppressed by the hipping pressure. The conversion to FeSi was 85% under 50 MPa, whereas it was 66% under 200 MPa when the reaction occurred at 1123 K for 15 min. The Jander equation was used to analyze the kinetics. The apparent activation energies were 129. 140 and 146 kJ/mol under the pressures of 50, 100 and 200 MPa, respectively. The activation volumes were also obtained as 53, 45, 49 and 43 ml/mol at 1023, 1073, 1123 and 1173 K, respectively. These volumes are relatively large compared with those of the homogeneous reactions. On the other hand, reaction (2) was accelerated by hipping pressure. The conversion to FeSi2 was 19% under 50 MPa, whereas it was 73% under 200 MPa when the reaction occurred at 1123 K for 120 min. The apparent activation energies based on the Jander equation were 660, 570 and 430 kJ/mol under the pressures of 50, 100 and 200 MPa, respectively. The activation volumes were - 180, - 190 and - 170 ml/mol at 1123, 1143 and 1173 K, respectively. These activation energies and volumes were apparent. However. it was concluded from the discussion about the contact area between grains that the reaction between Fe and Si was essentially affected by the hipping pressure.