The change in the particle size of whole milk powder during rehydration was studied by particle size analysis. This study also determined the effect of rehydration temperature (30, 40 and 50 ℃) and milk powder particle size on the rehydration process. Results showed that the rehydration of whole milk powder was affected by both temperature and its particle size. The rehydration time at 50 ℃ was 50% and less than 7.5% as compared to that at 40 ℃ and 30 ℃, respectively. For whole milk powder prepared using the same process, the larger the particle size, the shorter the rehydration time, and after rehydration under the same conditions whole milk powders with different particle size distributions exhibited different particle size distributions.