Poly(methyl methacrylate) (PMMA) gels have been prepared with radical polymerization by cross-linking methyl methacrylate monomer using ethylene glycol dimethacrylate as cross-linking monomer in toluene containing polystyrenes (PSs) with M w from 4000 to 400,000. The diffusion coefficients of the PSs in the PMMA gels swollen in deuterated chloroform have been measured by pulsed field-gradient (PFG) 1 H NMR method with the diffusing time Δ varied. From the experimental results, it is found that the network structure of PMMA gels prepared in the presence of PSs with M w =4000 and 400,000 are relatively homogeneous and inhomogeneous, respectively, within the Δ range from 40 to 500ms.