Abstract: Sheathed monochoria Monochoria vaginalis (Burm. F.) is a troublesome weed in rice (Oryza sativa L.) fields. Understanding damages of sheathed monochoria and economic threshold of control in rice fields could standardize sheathed monochoria control measures, promote weed control modernization and reduce herbicide pollution. A field experiment was conducted to study damages of sheathed monochoria to rice growth and yield along with economic threshold of prevention by manual weeding and herbicides use. The study set different densities (0 plant·m^-2, 1 plant·m^-2, 5 plant·m^-2, 10 plant·m^-2, 20 plant·m^-2, 40 plant·m^-2 and 80 plant·m^-2) of sheathed monochoria in rice field. The results showed that fresh biomass of sheathed monochoria at 80 plant·m^-2 was 17 t·hm^-2, with accumulated N, P, and K in weed shoots of 32.66 kg·hm^-2, 9.17 kg·hm^-2 and 58.17 kg·hm^-2, respectively. Rice was severely inhibited due to competition with sheathed monochoria for both nutrients and sun light. In comparison with the control, plant height, effective ear number and ear length of rice dropped by 20%, 46% and 11%, respectively at sheathed monochoria density of 80 plant·m^-2. However, blighted rice grain number increased by 3.5 folds, which resulted in rice yield drop by 55%. The optimum regression for rice yield loss rate (y) and sheathed monochoria density (x) was a square root model of y=8.437 1+5.024 9sqrt(x) at F=177.42 and P < 0.000 3. For 6 000~9 000 kg·hm^-2 rice yield, the sheathed monochoria economic threshold for manual weeding was 4.0~14.8 plant·m^-2. Also for 6 000~9 000 kg·hm^-2 rice yield, it was 1.0~1.6 plant·m^-2 for application of 10% Bensulfuron-methyl WP, 10% Pyrazosulfuron WP, 48% Bentazon and 56% MCPA sodium salts. The results suggested that sheathed monochoria severely inhibited rice growth and yield. Also chemical pesticides were efficient in controlling weeds based on estimated economic thresholds of control.