This paper describes results and analysis of experiments of compressive stiffness parallel to grain of glulam, with the focus on the damage zones near loading plates and joints. To investigate the influence factors of physical properties and the mechanism of the damage zone near the loading plates or the joints, compression tests on 90 specimens were performed. As a result, it was observed that damage zones exist near the loading plates and the joints. The Young’s modulus of the damage zone was approximately 2% of that of a common longitudinal modulus of elasticity. The lengths of the damage zone of wood-wood joints are larger than that of wood-steel joints. The length of the damage zone and its scatter increase as the width of the cross-section increases. In the specimens, the lengths of the damage zone were 0.7~4.7mm. Therefore, the length of the damage zone has a size effect of the cross-sectional area and depends on the processing accuracy on the contact surface. The size effect was evaluated by the strongest link model and the evaluation method of the compressive stiffness parallel to the grain was proposed.