Abstract: Objective To investigate the effect of ground state and singlet oxygen on the induction of HaCaT cell apoptosis and necrosis by 5-aminolevulinic acid-based photodynamic therapy （ALA-PDT）. Methods HaCaT cells were cultured and treated with various concentrations （0.5, 1.0, 2.0, 4.0 mmol/L） of ALA for 4 hours in the darkness, then irradiated with different doses （1.5, 3.0, 6.0, 12.0 J/cm2） of He-Ne laser. Twelve hours after the irradiation, MTT assay was used to measure the survival rates of the HaCaT cells, flow cytometry with PI staining to detect the cell apoptosis rate, and the level of intracellular reactive oxygen species （ROS） was determined by 2′,7′-dichlorofluorescin diacetate labeling. To mimic chemical hypoxia or singlet oxygen quenching in HaCaT cells before or after photodynamic reaction, the cells were pre-treated with various concentrations of cobalt chloride （CoCl2） or sodium azide （NaN3）, respectively, in addition to ALA-PDT. The changes in apoptosis and necrosis rates as well as ROS level in pretreated cells were compared with those without pretreatment. Results The rates of apoptosis and necrosis of HaCaT cells positively correlated with the concentrations of ALA and doses of He-Ne laser within the test range. The pretreatment with CoCl2 at 400 μmol/L significantly decreased the ROS level in cells, and the cell necrosis rate was downregulated from 57.65% ± 2.88% to 16.68% ± 1.86%, apoptosis rate from 43.80% to 15.40%. NaN3 of 10 mmol/L almost eliminated all the ROS from the cells induced by ALA-PDT, and enhanced the resistance of the cells to necrosis and apoptosis induced by ALA. Conclusion The alteration to intracellular ground state and singlet oxygen content can regulate the necrosis and apoptosis of HaCaT cells induced by ALA-PDT.