Chromochloris zofingiensis has been regarded as a potential producer of natural high-value bioproducts and is capable of accumulating secondary carotenoids under environmental stress conditions. Canthaxanthin is a secondary carotenoid that possesses powerful antioxidant activity and provides beneficial effects for human and animal health. Various means to improve carotenoid production have been employed including treatment of microalgal cultures with exogenous chemical treatments to instigate environmental stress conditions. However, information of exogenous chemical treatment on biomass and canthaxanthin production in C. zofingiensis is still limited. This study aimed to determine the effects of different concentrations of magnesium chloride (MgCl₂) on biomass and canthaxanthin accumulation in C. zofingiensis. The algal cells were cultured under the mixotrophic condition in the proteose medium supplemented with glucose 20 g L^-1 for 8 days. MgCl₂ was supplied into mixotrophic C. zofingiensis cultures at various doses of 2.5, 5, and 10 mM. Specific growth rate, cell density, and biomass concentration of each MgCl₂-treated culture were determined and the accumulation of canthaxanthin in C. zofingiensis was evaluated by HPLC analysis. The results revealed that MgCl₂ did not significantly inhibit C. zofingiensis growth rate. Under the mixotrophic condition, the cell density of C. zofingiensis was greatly increased on day 8 of cultivation compared to day 0. MgCl₂ treatment at 5 and 10 mM could considerably induce canthaxanthin accumulation in C. zofingiensis by about 28.87-42.40% compared to control, in which the highest canthaxanthin content of 0.937 mg g^-1 was found upon treatment with 5 mM MgCl₂, whereas low concentration at 2.5 mM gave slightly lower canthaxanthin content than the control. The results of this study thus have an implication for the development of the strategy to improve canthaxanthin accumulation in C. zofingiensis.