Poly(methyl methacrylate) (PMMA) with various degrees of cross-linking were prepared from methyl methacrylate and a cross-linker, and the effect of dilution of the polymerizable mixture by a thermoplastic PMMA on the cross-linked PMMAs was evaluated. The rheological properties were characterized in linear viscoelasticity and in uniaxial extensional flow. A critical gel is formed at concentrations of the cross-linking agent neopentyl glycol dimethacrylate (NPG) of approximately 250 mol ppm both in the case of PMMAs, which are not diluted by an addition of thermoplastic PMMA to the monomer (Recipe-A), and of PMMAs, which were obtained by an addition of 25 wt% low molecular weight thermoplastic PMMA to the monomer (Recipe-B). Significant strain hardening is observed for concentrations of NPG at and above 100 mol ppm for PMMAs based on Recipe-A and for all PMMAs produced by Recipe-B. At the same NPG concentration of 30 mol ppm, PMMA produced by Recipe-A shows very little strain hardening, while PMMA produced by Recipe-B shows significant strain hardening. This is due to the difference in the molecular weight distribution: PMMA from Recipe-A is mono-modal with M (w) /M (n) = 2.5, while PMMA from Recipe-B is bimodal with M (w) /M (n) = 5.6. Surprisingly, the strain-hardening tendency is strongly increasing with increasing NPG concentration, and at the same NPG concentration, the strain hardening of PMMAs produced by Recipe-B is higher than that of PMMAs produced by Recipe-A. This difference can be attributed to the dilution effect of the (unreacted) thermoplastic PMMA in Recipe-B PMMAs. The elongational flow behavior was also analyzed by the Molecular Stress Function (MSF) model.