Abstract: In order to study the influence of brown-rot decay on main properties of hardwood, Populus cathayana sapwood specimens (20 mm×20 mm×300 mm) were made and brown-rot decay was incubated on the specimens in laboratory with brown-rot fungus Gloeophyllum trabeum for 12 weeks. The mechanical properties, microstructure and chemical compositions of both healthy wood and decayed wood were investigated once a week. And then changes of all indexes during decay processing and the correlation between mechanical properties and chemical compositions were contrastively analyzed. The research results showed that more and more hyphae were found in wood cell lumina under scanning electron microscopy (SEM) with decay degree increasing. Pit membrane and the cell wall on the edge of pit were successively broken up at a wood weight loss of 10% and 16% respectively. Finally, the cell wall was ulcerated seriously while the wood weight loss increased to 24%. Both degradation time and weight loss had extremely significant influence on wood mechanical properties (P0.01). The loss of impact bending strength (IBS) and modulus of rupture (MOR) increased in forms of logarithmic function with decay degree changing (R2 = 0.922**,0.830**), while the loss of modulus of elasticity (MOE) and the compressive strength parallel to grain (CSⅡ) increased slowly in linear trends (R2 = 0.991**,0.986**). Whether the response speed to decay or the influenced degree by decay, the ranking of four mechanical properties were IBSMORMOECSⅡ. Holocellulose, hemicellulose and extractions of specimens varied significantly at different decay degrees (P0.01), but cellulose and lignin had no significant variance (P0.05). During the decay processing, hemicellulose was first and mainly degraded by brown-rot fungus. The prominent degradation of cellulose began when the weight loss was about 20%. A strong relationship between the loss of wood IBS and the degradation of hemicellulose was found. The variation of MOR was dependent on holocellulose content. And the linear decreasing trends of MOE and CSⅡ were determined by cellulose’s slow degradation. In conclusion, during the brown-rot decay processing, the degradation of chemical compositions and the structural failure of wood cell wall at the micro level fundamentally resulted in the decrease of macroscopical mechanical properties.