The load-bearing ability of dental restorative materials under cyclic high-stress applications depends upon mechanical properties established by the composition and microstructure. The microstructure and the elastic properties of neat resin and two resin-based glass-reinforced composites have been studied. The microstructure of these materials has been examined using x-ray diffractometry (XRD), scanning electron microscopy (SEM) including energy dispersive spectrometry (EDS). The elastic properties, i.e., Young's, shear and bulk moduli and Poisson's ratio were determined from ultrasonic velocities and densities. The ultrasonic velocities were measured using a pulse-through transmission method; density was measured using a buoyant force method. These studies showed that: (1) these materials have amorphous structures; (2) these materials have Young's moduli of the order of 20 GPa, and (3) the silane couplant apparently did not significantly affect the elastic properties of these resin-based composites.