The effects of brown-rot decay on the mechanical and chemical properties of poplar (Populus cathayana Rehd(I)) wood were investigated, while the correlation of several properties to the decay degree and to each other were quantified. During the decay process, the losses in impact bending strength (IBS) and modulus of rupture (MOR) increased logarithmically with the degree of decay, whereas the losses in modulus of elasticity (MOE) and compressive strength parallel to grain (CSI) increased slowly with linear trends. The ranking of the four mechanical parameters for the response speed to decay and the degree of the influence of decay were IBS >MOR >MOE >CSI. Hemicelluloses were preferably decomposed by the brown-rot fungus. The prominent degradation of cellulose began from a weight loss (W-L) of 18.7%, where the relative crystallinity decreased. In summary, the degradation of hemicelluloses caused a fast and significant decrease in IBS while the subsequent removal of paracrystalline cellulose resulted in MOR loss. The variation of MOE was in correlation with the cellulose content. A decrease in CSI was due to the slow deterioration of crystalline cellulose.