Several reports of applications utlizing a cristobalite, one of the eight crystal forms of silica, began to appear over the past dozen years. In these reports, a cristobalite is used as a filler of a composite material,however mechanical properties of cristobalite filled hard resin have not been reported. Therefore, the flexural properties of a cristobalite spherical particle-filled epoxy resin was investigated in this research. First, the flexural modulus of an amorphous silica spherical particle-filled epoxy resin (A-SiO₂/EP) and a cristobalite spherical particle-filled epoxy resin (C-SiO₂/EP) were equal and both were higher than that of their matrix (epoxy resin: EP). Moreover their values increased as the volume fraction of filler increased. Futhermore the measured values were higher than the theoretical values, however the tendency were largely in agreement. Theoretically, the elastic modulus of the C-SiO₂/EP is almost equal to that of the A-SiO₂/EP up to approximately 90 vol%. Next, the flexural strength of the A-SiO₂/EP and the C-SiO₂/EP were equal and both were lower than that of the EP. Moreover their values decreased as the volume fraction of filler increased. Since the measured values were lower than the predicted value of perfect adhesion and higher than the predicted value of non-adhesion, the interfacial adhesion is between perfect adhesion and non-adhesion. The influence of thermal stress on the flexural strength of the A-SiO₂/EP and the C-SiO₂/EP were equal and slight. Thus, it was revealed that the flexural properties of the C-SiO₂/EP equals to that of the A-SiO₂/EP. From the viewpoint of mechanical property, this research showed a possibility of a cristobalite as a new filler for a composite material.