The deformation behaviour of rubber toughened poly(methyl methacrylate) (PMMA) was studied using real time small angle X-ray scattering (SAXS) with a synchrotron radiation source. The invariant analysis method was used to analyze the influence of rubber particle concentration and rubber cross-linking density on craze concentration quantitatively. It is shown that both rubber particle concentration and the cross-linking density of the rubber layer have a significant effect on craze density. The craze density increases as the concentration of rubber particles increases from 10 wt % up to 30 wt %; thereafter, it decreases as the concentration of rubber particles increases further. For all polymer matrices with different cross-linking densities, a maximum craze concentration appears at similar to 30 wt % of rubber particle, which corresponds to a true rubber concentration of about 15 wt %. The influence of cross-linking density on craze concentration is similar to that of rubber particle concentration. As the cross-linking density increases, the craze density first increases and then decreases. The appearance of crazing coincides with the occurrence of the macroscopic yield point of the polymer sample, corresponding to the beginning of plastic deformation. It appears that a sufficient concentration of crazes is needed to achieve an optimum toughness of the PMMA matrix. For the same cross-linking density of the rubber, high toughness requires a sufficient rubber particle concentration which can generate many crazes, while, for a given rubber particle concentration, high toughness is associated with a lower craze density.