Silicon nitride specimens embedded with a single spherical void were prepared for a flexural strength test. The measured flexural strengths of the specimens were compared with theoretically estimated strengths. Estimation of the strengths was done using a Gibbs free-energy criterion. The energy was calculated by Eshelby’s equivalent inclusion method for a specimen with an embedded void. Good correspondence was obtained between the experimental and the estimated fracture loads. A deviation of the estimated strength from the experimental value was observed for voids whose diameters were comparable with intrinsic defects.