Objective: To elucidate pathophysiology of melanocytorrhagy by investigating morphological alterations of dendrites and intracellular calcium concentration(Ca2+) of human melanocytes induced by hydrogen perioxide(H2O2) in vitro. Methods:Melanocytes were cultured routinely, and 0.1% and 0.05% H2O2 were added to culture medium respectively. Dendrites’ length was observed at 30,60,90,120 and 150 min after treatment,and measured accurately by microscope optical ruler. Intracellular calcium was detected with Flu-3/AM probe by flow cytometry. Results:H2O2 induced a notable retraction of dendrites,of which concentration 0.1% had more potent effect than 0.05%. Intracellular calcium was up-regulated with the inducement of H2O2 detected by Flu-3/AM probe. Calcium peak presented was at 30 min after treatment, which is earlier than the first retraction peak of dendrites at 60 min. Conclusion:The results suggest retraction of dendrites might be related to accumulation of intracellular calcium, which led to failure of reflux of calcium.