Research Highlights: Orthotropic tension behaviors of poplar and Chinese fir were investigated at a wide relative humidity (RH) range. Background and Objectives: Poplar and Chinese fir are typical plantation tree species in China. Mechanical properties of plantation-grown wood varies from naturally-grown one. To utilize poplar and Chinese fir woods efficiently, fully understanding their moisture content (MC) and orthotropic dependency on tension abilities is necessary. Materials and Methods: Plantation poplar and Chinese fir wood specimens were prepared and conditioned in series RH levels (0–100%). Tensile modulus (E) and strength (σ) were tested in longitudinal (L), radial (R), and tangential (T) directions. Results: The E and σ results in transverse directions confirmed the general influence of the MC that decreased with increasing MC. However, both E and σ in L direction showed a trend that increased at first, and then decreased when MC increased. The local maximums of stiffness and strength may be associated with straightened non-crystalline cellulose, induced by the penetration of water into the wood cell wall. Using the visualization method for compliance, the tension abilities of poplar and Chinese fir exhibited clear moisture and orthotropic dependency. Conclusion: Both poplar and Chinese fir showed a significantly higher degree of anisotropy in the L, R, and T directions. The results in this study provided first-hand data for wooden construction and wood drying.