Abstract: Objective To study the effect of plasma calcium concentration on aggregation of platelets. Methods Different amounts of CaCl2 were added into venous blood samples taken from healthy volunteers（n=42）.Aggregation of platelets was detected by plasma turbidimetry. Results when the plasma calcium concentration was 2.11-0.12mmol/L,the aggregation rate of platelets induced by adenosine diphosphate（ADP） and arachidonic acid（AA） was（75.9±10.4）%-（51.8±9.6）% and（83.7±13.9）%-（64.4±12.2）%,respectively（P＜0.01）,and then decreased with the decreasing concentration of Ca2+.When the plasma calcium concentration was 2.11-33.66mmol/ L,the aggregation rate of platelets induced by ADP and AA was（75.9±10.4）%-（94.7±4.8）% and（83.7±13.9）%-（93.2±5.5）%,respectively（P＜0.01）,and then increased with the increasing concentration of Ca2+.When the plasma calcium concentration was 39.00 mmol/ L,the aggregation rate of platelets induced by ADP and AA was 9.1%±5.3% and 11.1%±4.5%,respectively（P＜0.01）. Conclusion Plasma calcium concentration has a significant effect on aggregation of platelets.When the physiological concentration of Ca2+ is lower than 2.1mmol/L,the aggregation rate of platelets decreases with the decreasing concentration of Ca2+.When the physiological concentration of Ca2+ is higher than 2.1mmol/L,the aggregation rate of platelets increases with the increasing concentration of Ca2+.The aggregation of platelets is inhibited when the plasma calcium concentration is≥39.0 mmol/L.