The interrelationship between processing conditions, developed microstructure and mechanical properties has been studied for a series of injection moulded rubber-toughened poly(methyl methacrylate) (RTPMMA) samples. A design of experiments (DOE) approach has been adopted to investigate the effect of barrel temperature, mould temperature, screw speed and back pressure on the mechanical properties of the mouldings. The back pressure has been identified as the single most important factor affecting the sample properties. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) have been used to study the relationship between the rubber-toughened microstructure and mechanical properties. SEM has shown that the rubber particles are almost spherical in the central region of the injection-moulded samples, away from the sample surface. AFM topography measurements combined with UFM can reveal the distribution, elongation and orientation of the rubber particles close to the surface of the sample. UFM in particular reveals the core-shell structure of the particles as well as the presence of particles immediately under the surface, invisible by AFM. The particles are elongated in the skin region of the injection moulded samples and well aligned to the melt flow. UFM has shown that samples with different flexural properties exhibit a difference in the number and distribution of rubber particles present in the skin region.