This study evaluated the effects of treatment type of reed (Arundo donax) particles bonded with urea-formaldehyde resin in panels, press closing time, and amount of almond shell powder used as an extender in the glue line had on the modulus of rupture (MOR) of sandwich panels in dry and wet conditions. The differences between the bending strength parallel and perpendicular to the face/back veneer grain in dry (MOR||dry and MOR⊥dry) and wet (MOR||wet and MOR⊥wet) conditions were determined using the t test. According to the results, the selected variables significantly affected the bending strength. The highest dry and wet MORs were obtained by using hot-water–leached particles in the core layer, increasing the press closing time, and increasing the extender content in the glue line. According to the t test, there was a statistically significant difference between the t values of MOR||dry and MOR⊥dry and also between MOR||wet and MOR⊥wet. The maximum MOR||dry and MOR||wet (15.33 and 13.13 N/mm2, respectively) were observed to be higher than those of MOR⊥dry and MOR⊥wet (13.37 and 12.41 N/mm2, respectively).