Differentiating from normal wood, baeterial of water drop shape, flake shape and lump shape are observed in pit membrane in wetwood of Abies lasiocarj pa (Hook) Nutt. (sub-alpine fir) using scanning electronic microscope. Not only ray parenchyma cells in wetwood are partially degraded but also margo strands in pit membranes are somewhat degraded by bacterial activities. Most of bordered-pit membranes in normal wood are un-aspirated at green condition and the proportions of aspirated pits in early wood and latewood are only 6.8% and 13.4% respectively. Nevertheless, most of bordered-pit membranes in wetwood are aspirated at green condition and the proportions of aspirated pits in early wood and latewood are at 77.7% and 72.1% respectively. The problem of hard-to-dry for sub-alpine fir could be reasoned from the considerable amount of aspirated pit membranes in wetwood.