Functional materials such as rare earth - transition metal intermetallic compounds can easily be produced from the mixtures of rare earth oxide, transition metal. and calcium by the reduction-diffusion (RD) process. The formation rates of rare earth (R = Ce, Pr, Nd or Sm) - iron intermetallic compounds by the RD process were measured: the thickness of R2Fe17 compounds layer formed on Fe plate immersed in R-Fe melts of which compositions were on their Fe-side liquidus lines. The experiment was carried out in an Ar atmosphere at the temperature range from 1000 to 1300 K. It was tried to discuss the measured formation rates based on the energy fluctuations of systems. The energy fluctuations were calculated by the extended Huckel method for two types of model clusters: an alloy cluster containing 89 atoms and R2Fe17 type compound cluster containing 87 atoms. The energy fluctuations of the alloy clusters increase with the concentration of each rare earth in the clusters and a maximum was observed at about 11 at.% rare earth, which corresponds to the composition of R2Fe17. The temperature dependence of the formation rates of R2Fe17 compounds was discussed with the calculated energy fluctuations.