Changes of organic carbon after the entrance into dam reservoirs were investigated using water samples collected in May September and October in 2010 from the inflow sites and the outlets of four selected dam reservoirs-Soyang Chungju Chungju regulation and Uiam. Increase of refractory disslolved organic carbon was observed only for large dam reservoirs with long residence times whereas the trend was not found for relatively small reservoirs. The effects of residence times on organic carbon changes were further confirmed by significant postitive correlations between monthly residence times and the relative increase of either dissolved organic carbon or R-DOC concentrations. Comparison of spectroscopic characteristics of DOC revealed that the changes in large reservoirs in May might result from in-lake processes. The inflow of terrestrial sources of DOM during storms appears to largely affect the DOC quality of the large reservoirs for the rest of the sampling periods. The mechanism however did not fully explain the behaviors of DOC for the small sized reservoirs. Our combined results suggested that both residence time and the input of allochthonous carbon sources might substantially influence the quantity of DOC as well as its quality in dam reservoirs.