The rate-controlling step of resoluting N-acetyl-D
L-alanine with porous immobilized Aspergillus Oryzae cells was studied. In the batch reaction with high ratio of substrate to catalysts
the rate ?controlling step was determined by measuring the concentration variation of produced L-alanine in solution. The results showed that the substrate concentration and temperature were important factors which influenced the rate - controlling step. The results of model calculated with effective diffusion coefficient obtained were in agreement with experiment data.