Abstract: Like the greenhouse effect, ocean acidification (OA) is an environmental problem caused by increased emissions of anthropogenic carbon dioxide (CO2). The reduced pH of the seawater will seriously affect the growth, development and reproduction of marine organism, and thus threaten the marine ecosystem. There have been great concerns about the impact of heavy metal pollution on the marine biological and ecological system in recent years. However, up to date little is known about the combined effects of ocean acidification and heavy metal pollution on shellfish. Using artificial simulation of ocean acidification and semi static acute toxicity test we investigated whether ocean acidification would affect the acute toxicity of Cd2+ and Hg2+ on juvenile M. lamarckii. Experiments were carried out at pH 8.20, 7.80, 7.60 and 7.40 respectively. We found that although ocean acidification alone did not change the survival rate, it significantly increased the toxicity of Cd2+ and Hg2+ to juvenile M. lamarckii compared to the control. Moreover the to-xicity of Cd2+ and Hg2+ rose as the acidity of seawater increased, and the two heavy metal ions were the most toxic at the lowest experimental pH 7.4. The corresponding 96h LC50 concentrations for Cd2+ and Hg2+ were 4.068 and 10.332 mg/L respectively. As the pH of seawater increased the toxicity was reduced in other experimental trials. For Cd2+, the 96h LC50 concentrations were 4.728 mg/L at pH 7.6, 5.947 mg/L at pH 7.8, and 6.458 mg/L at pH 8.2, and the 96h LC50 for Hg2+ were 10.595, 11.169 and 12.027 mg/L respectively. Compared to the control, the toxicity of Cd2+ and Hg2+ on juvenile M. lamarckii increased by about 1.4 and 1.2 times respectively at lower pH. Our results shed lights on the inte-ractive effects between ocean acidification and heavy metal pollution, therefore would contribute to the protection and restoration of M. lamarckii population.