Abstract: Hydrological conditions of the Han River display obvious change with constructingdams for the hydroelectric stations. In order to show how far the spawning of fishes isaffected from the change of the hydrological conlition, an ecological investigation wascarried out in 1976 to 1978. The results are summarized as follows. 1) There are 25 species of fishes which cast their "drifting" eggs in the HanRiver. The most common ones are Ctenopharyngodon idellus, Mylopharyngodon piceus,Hypophthalmichthys molitrix, Aristichthys nobilis, Elopichthys bambusa, Ochetobiusclongatus, Leuciobrama macrocephalus, Parabramis pekinensis, Squaliobarbus curriculus,Corcius heterodon, Erythroculter spp., Rhinogobio spp., Siniperca spp., etc. Nearly allof them are commercial fishes. 2) The water temperature and current velocity of the spawning ground seem to bethe essential factors for fish spawning. The commercial fishes begin to spawn at thewater temperature of 16-20℃, while the four farm-fishes above 18℃. The increaseof the current velocity may be regarded as the essential factor that induces the fishes tospawn but these seem to have differences between different species of fishes. 3) Since the establishment of Danjiang Reservoir, there are ten spawning groundsin the upper course of the Han River, seven in the middle course and in its branch, theTangbai River, have been found. The amount of the eggs laid during the whole breedingseason are calculated at 174590 million in the upper course in the year 1977 and about 4700million in the middle course of the year 1976. The eggs laid by the four farm-fishesare calculated at about 17127 million in the upper course and 230 million in the middle. 4) In the lower part of the upper course of the Han River the better conditionof spawning grounds provides many spawning grounds for "drifting" eggs fishes tospawn, but the eggs soon pass into the stagnant water of the reservoir where the eggs pos-sibly sink to the bottom then make a serious damage in the development of the embryos. Inthe middle course of the river, hydrological condition may be highly altered owingto the presence of the reservoir, the water flow out from the hydroelectric station beingcolder and thus cause the spawning ground reduced in area and much retarded. 5) Because of the habits and habitats of the fishes which live in the drainage ofthe Han River, it seems that it is unnessary to install any kind of fishpass on thedam of the reservoir.