In situ refractive index technique was used to study the gelation process in the free-radical cross-linking copolymerization (RCC) of N,N-dimethylacrylamide with N,N-l-methylenebisacrylamide as the cross-linker. The change of refractive index was measured and used to monitor the gelation process. The influence of different polymerization conditions on the gelation process, such as monomer, cross-linker and initiator concentrations was investigated and discussed in detail. The kinetic model under steady-state conditions was employed to determine the monomer consumption rate during the gelation process.