In Bibai peatland in Hokkaido, Japan, the decomposition of peat is enhanced due to the drying tendency of the soil. Accordingly, the community of Sasa senanensis invades the community of Sphagnum, whose area is diminishing rapidly. Peat samples were collected from this peatland on a transect line which passes through the original Sphagnum community as well as the invading Sasa community. Several physical and chemical properties and indices for the degree of decomposition of peat were determined for these samples. In accordance with the drying tendency, the degree of decomposition and humification increased remarkably as revealed by the change in total nitrogen content, C/N ratio, ash content, fiber content by authentic and new methods, degree of humification by the method of KAILA as determined at 550 and 340 nm, field moisture content, and bulk density. Highly significant correlations were also observed between these characteristics. The new method for the determination of fiber content as proposed in this paper is easy and rapid, and gave almost the same results as those which were obtained by the authentic rubbed fiber method. No significant difference was observed between the results obtained by these two methods.