Ten levels of rice defoliation (0.10,20,30,40,50,60,70,80 and 90%) were done at tittering stage or flowering stage to simulate damage caused by rice leaf miner, Hydrellia prosternalis Deem. Grain yield and its components were determined. Total grain yield and its components were negatively affected by defoliation in both tittering and flowering stages. Defoliation at flowering stage was more negatively effective on grain yield than that at tillering stage. In tittering stage, there was no significant loss in yield before 90% defoliation, and did not occur before 60% in flowering stage. This study presents data which help determining the economic threshold for insect control through costs of yield loss against insectici‌dal control which changes from one season to another. As an important conclusion, insecticidal control may not be ur-gent against this insect in most cases because 1/5 of leaves in tillering stage or 2/5 in flowering is enoguh to produce yield without significant toss.