Field experiments were conducted to investigate rice - Echinochloa crus-galli and rice - Eleocharis kuroguwai competition under transplanted rice cultivation in four major rice production areas; Suwon, Daejeon, Iksan, and Naju in Korea. Rice yield data were used to predict rice yield as a function of plant densities of E. crus-galli and E. kuroguwai using a rectangular hyperbola and to determine economic threshold (ET) levels of the weeds. Both weed species significantly reduced number of tillers at early rice growth stage, resulting in significant reduction in number of spikes, and the other yield components such as number of grains, maturity and 1,000-grain weight at later growth stage. The weed competitivity represented by parameter ranged from 0.0145 to 0.0346 for E. crus-galli and from 0.0037 to 0.0187 for E. kuroguwai, indicating that the competition effect of E. crus-galli on rice yield was slightly greater than that of E. kuroguwai. The ET values of E. crus-galli were between 0.298 and 1.078 plants $m^{-2}$, while those of E. kuroguwai were between 0.848 and 5.298 plants $m^{-2}$, depending on weed competitivity and herbicide price. Therefore, our results can be used to support decision-making on herbicide application for E. crus-galli and E. kuroguwai management in transplanted rice cultivation.