Abstract: Stress-strain relationship of longitudinal compressed hydrothermal-microwave treated juvenile and mature northeast China ash wood and their chemical composition variation were analyzed. The stress-strain constitutive relation of longitudinal compressed wood were established. Results showed that at the beginning of compression, the curves followed the linear Hooke’s Law. Meanwhile, the curves primarily fit the linear mechanics relation at the elastic-plastic region. The founded stress-strain constitutive relation showed that both juvenile and mature wood longitudinal compression could be expressed by an equation consisting of a constant term(proportional ultimate stress)and stress-strain linear relationship. The higher modulus of elasticity(MOE)and stress of juvenile wood was caused mainly by the difference of wood components between juvenile and mature ash wood at the same longitudinal compression rate.