Mechanical properties of lumber are affected by several factors. It is known that decreasing moisture content (MC) led to increase the boards’ stiffness. That means MC has an impact on the modulus of elasticity and the modulus of rupture. In North America, lumber stiffness may be evaluated using mechanical stress rating (MSR). The purpose of the present study is to propose a MOE / MC correction factor for Eastern Black Spruce that can improve the MSR Tool Kit and to assess the impact of MC and MC gradient on MSR production yield. An adopted MOE / MC correction factor for Black Spruce was developed but this factor is not significantly different from the one proposed by ASTM standards. Decreasing MC increase lumber MOE and MOR but results in visual downgrading. MC distribution maximising MSR yield was found to be 16% with relatively small standard deviation (1.5 to 2.5%). High MC gradient tends to increase board stiffness and thus lumber with higher MC gradient yielded more MSR grades.