Abstract: A compression shear test jig for determining the internal shear strength of the weakest plane of a wood fiber or particle panel product. The test jig comprises an upper and lower loading head adapted to be used in conjunction with a universal testing machine or press. The upper and lower loading heads are each provided with a sample engaging face disposed at substantially a 45.degree. angle. These faces are in parallel spaced apart relation. The bottom end of the lower face and top end of the upper face are each provided with a sample restraining element. The lower head body includes a base which rests upon a low friction surface permitting unrestrained lateral movement to occur as shear strain develops. Preferably the upper loading head includes a pivoted cap so that compressive force is applied uniformly. The sample engaging faces are preferably formed in separable face plates which are adjustable relative to the loading head bodies to compensate for samples of different thicknesses.