Abstract: To study impacts of fresh water on morphology and structure of mitochondrion-rich cells（MRCs） of Epinephelus coioides juvenile under the stress of acute fresh water， the osmoregulatory function of MRCs and the structural changes in gill mitochondrion-rich cells were examined with LM in intervals of 2， 6， 12 and 48 h and with scanning electron microscope in 48 h. Results showed that the short and long diameter of MRCs in fresh water were significantly shorter than that in the control group except that the diameter of gill lamellaes were widened in fresh water （p<0.05）. There is a special apical membrane in gill MRCs. The apical surfaces of gill MRCs in fresh water were generally flat with dense microvilli-like projections. In contrast， the gill MRCs in the control exhibited deeply invaginated surfaces with smaller orifices. As results， under acute fresh water stress the structure of gill MRCs were changed and its osmoregulatory function was impaired. As consequence E. coioides juvenile cannot survive for a long time when transferred from seawater to fresh water with no acclimation.