An open-label clinical trial was performed to test the effects of unicellular green alga Chlorella supplementation on mercury concentrations of hair and blood in healthy subjects. Fifty-eight healthy participants (36 male and 22 female) were assigned to Chlorella and control groups. The Chlorella group of 35 subjects received Chlorella tablets (9 g/day) for an experimental period of 3 months while the control group of 23 subjects did not. Total mercury concentrations of hair and blood were analyzed at the beginning and end of the experimental period for estimation of methylmercury (MeHg) levels in the body. The hair mercury concentration of the Chlorella group (n = 33) was significantly decreased during the experimental period (p = 0.041) while the change in the control group (n = 23) was not significant (p = 0.362). Although the decrease in blood mercury concentration in the Chlorella group (n = 19) was not significant (p = 0.084), the change of values (values at end – values at beginning) in this group was significantly greater than that in the control group (n = 20, p = 0.038). The fish intake rates remained relatively constant during the experimental period in both the Chlorella and control groups. These results suggest that supplementation with Chlorella for 3 months in healthy subjects might reduce their body MeHg levels.