Although several studies have shown that the administration of $17{eta}$-estradiol (estrogen) is cardioprotective to ischemia-reperfusion (I/R), the molecular mechanisms are largely unknown. Therefore, we investigated the effects of estrogen on myocardial I/R injury in rat that were sham operated (Sham), ovariectomized (OVX), or ovariectomized and then given estrogen supplementation (OE). Langendorff-perfused rat hearts were subjected to I/R stimuli and the effects of estrogen were examined on cardiac performance. Additionally, we examined the mechanism of estrogen-mediated inhibition of apoptosis. Depression in cardiac contractile function and an increment of calpain activity were observed during I/R in the OVX rats. Estrogen replacement recovered cardiac contractile function and attenuated calpain activity, Bid cleavage, and caspases activities. Through in vitro assay using cardiomyocytes, we demonstrated that addition of $H_2O_2;(100{mu}M)$ significantly increased calpain activity, which was attenuated by estrogen. Moreover, calpain activity was inhibited by calpain inhibitors such as ALLN or leupeptin, but not by caspase-8 inhibitor peptide. These results suggest that estrogen protects the heart against I/R injury through the decrease of calpain activity, Bid cleavage and caspase-8 activity. These apoptotic mechanisms may playa critical role on I/R-associated cardiac damage.