Abstract: To investigate the hazards of Hydra robusta to fish fry of Coilia mystus at the earlier development stage,we designed an experiment to put a fish fry (1-7 days of age) in the cultured solution of 200 milliliter contained 1-5 polyps.Results of experimental data analyzed by two-way analysis of variance showed that:With the polyps density increased,the death rate of fish fry increased (up to died) and the average weight of fish fry was decreased significantly (P<0.01).With the age of fish fry increased,the death rate was decreased and the average weight was increased significantly (P<0.01).It is concluded that the hazards of Hydra robusta to fish fry of Coilia mystus is extremely serious,but becomes lower and lower with development of fish fry,however,it finally influences the increase in fish weight.