OBJECTIVE To study the protective effect of rosmarinic acid (Ros A) on the primary cardiomyocyte hypoxia/reoxygenation (H/R) injury. METHODS Primary cardiomyocytes of rats were cultured in vitro to establish the H/R injury of cardiomyocytes and observe the changes in the cell viability and LDH leakage. The changes in ATP content and ROS in cardiomyocytes were measured by using chemiluminescence and fluorescent probe technique. The effects of rosmarinic acid on the apoptosis of cardiomyocytes, cleaved-caspase 3, Akt and p-Akt protein expression were further detected by flow cytometry and western blot analysis. RESULTS According to the experimental results, Ros A at doses of 25, 50, 100 mg x L(-1) could inhibit the decrease in H/R-induced cell viability, LDH leakage and excessive ROS generation, and maintain the ATP level in cells. Ros A at doses of 50, 100 mg x L(-1) could remarkably inhibit the H/R-induced cardiomyocyte apoptosis and down-regulate the expression of cleaved caspase-3. Moreover, Ros A at doses of 100 mg x L(-1) could significantly up-regulate the expression of p-Akt. CONCLUSIONS Ros A has the significant effect in resisting the cardiomyocyte H/R injury, improve cardiomyocyte energy metabolism and reduce cell apoptosis, which is related to the activation of Akt pathway.