Abstract: In this study, the catalysts with three kinds of particle size (1.0 mm, 2.0 mm and 4.0 mm) were fabricated to explore the size effect for hydrogen isotopes separation. It is found that the performance of the catalysts significantly decreases with the increasing size of the particles prepared using the same process. Furthermore, the optimal reduction temperature was identified for three kinds of catalysts to decrease the size effect. When the particle sizes increase from 1.0 mm to 4.0 mm, the reduction temperatures increase from 240 ℃ to 320 ℃. In addition, the TPR (temperature programmed reduction) approach was deployed to investigate the underlying mechanism for the effect of reduction temperature. The results indicate that the higher reduction temperature maybe improve the transfer of heat and mass during the reduction process.