The purpose of this study is to investigate the benthic population and structural change of river beds where improvement works were conducted at different periods. The changes of periphyton and benthos densities were determined at the resectioned river beds from autumn to winter. The survey results suggested that structural divesity of river bed was dependent on length of period after the improvement works. River bed has larger variety of structure with longer period after resectioning works. There was a difference in benthic population and its density between block-type beds with and without deposited stones and sands. However there was not a remarkable difference in periphyton and benthos densities between block-type bed and stone-type bed, once stones have transported and deposited to some extent on the block-type bed. River bed points with larger biomass density had a tendency to have lower diversity index values, because biomass amount was deeply dependent on the accumulation of one or two predominant biomass groups.