The aim of the present investigation was to elucidate the ecological succession of plankton in soyang multiple-purposes reservoir which was constructed on Autumn 1972, during the early stages of impounding. The environmental factors water temperature, dissolved oxygen, pH, transparency and some other factors were surveyed, and then fauna of plankton were studied at the same time. The results obtained from this investigation are as folioms: 1) As for the vertical distribution of water temperature, it was relatively wide as ranging 6.4℃∼8.3℃ from surface to bottom, and the patterns of summer stagnation period was slighthy appeared. 2) The transparency was in general higher value than that of the other reservoirs. The reason that the value was higher in summer is because of low amount of seston as in progress of impounding. 3) No vertical changes of dissolved oxygen is, but in general over saturated states or 80% of dissolved oxygen is occurred in April and May, except 50% of September. It is suggested that the phenomena is one of the characteristics of the river type. 4) The patterns of ecological succession of plankton according to the months sampled are as follows: P. trigla (March)→A. priodonta (April)→C. hippocrepsis (May) →D. brachyurum (June)→D. longispina, D. divergens (July)→D. longispina, D. corona (August)→C. guadrangula, B. longirostris (September)→B. longirostris, T. taihokuensis (October)→Tabellaria sp., Mougeatia sp. (November)→Tabellaria sp., C. strenuus (December). 5) By the results mentioned above, only zooplankton was sampled and identified, and phytoplankton was very rare. Through March to August, but phytoplankton initiated to appear after September. This is an evidence that newly species inflowing to the reservoir are increased in speedly. The heleoplankton (pond-type plankton) was the noticeable species in the reservoir, so it is concluded that the species was derived from storage ponds. 6) It is assumed that the two specles of T. taihokuensis and A. pacificus of Copepodid among the identified species will be specially interested at a point of ecological view in future.