Mass [i.e., linear polystyrene (PS) probe] transports in polyethyl methacrylate (PEMA) gels with ethylene dimethacrylate (EDMA) as crosslinker were studied by dynamic light scattering (DLS). For PEMA gels in toluene, we have established that it is a very well matched system, so that it is very convenient to study the mass transport of the PS macromolecule in such a system. Experiments show that the obstruction effects of PEMA gels for the mass transport of PS are obvious. Reasonably, the lower the gel obstruction for mass transport, the higher the diffusion coefficient D-t of PS in gel. Moreover, the larger the molecular weight of PS, the greater the obstruction effect of the gel. Furthermore, we traced quantitatively the evolution of D-t of PS during the crosslinking copolymerization of PEMA. The results show that, after the reaction, the obstacle for mass transport in the gel develops gradually.