Abstract: To understand the biodegradation processes of rice straw in compostiong,samples of the straw were taken at different composing stages for observation on the environmental scanning electron microscope(ESEM) to explore distribution of microorganisms in the straw and their effects in decomposing the straw from the angle of mechanical action.Results show that no microorganisms were found on the surface of raw straw,and they began to appear on the surface of straw at the initial stage of composting.With the composting proceeding,the microorganisms grew dense on the surface and were also found in cracks formed on the straw in decomposing.Microorganisms participating in decomposing of straw were distributed in colonies,and dominated with cocci and hyphomycete,which varied sharply in individual size.The observation reveals that large-sized hyphomycetes intrude into cracks formed in the straw being composted creating mechanical action that helps break down the straw.