Schisandrin B (Sch B), an active composition isolated from the fruit of Schisandra chinensis, has been proved to possess antiinflammatory, antioxidant and anti-endoplasmic reticulum (ER) stress effects in many rodent tissues. However, the exact mechanism of cardioprotective effect of Sch B still needs more study. Here, we detected the effects of Sch B on myocardial ischemia/reperfusion (I/R) injury rats. I/R injury model in this study was established by left anterior descending coronary artery ligation for 40 min followed by 1 h of reperfusion. Male healthy rats were randomly divided into five groups: the sham, I/R, Sch B (20 mg/kg) + I/R, and Sch B (40 mg/kg) + I/R, Sch B (80 mg/kg) + I/R, with 10 rats in each group. We showed that Sch B treatment significantly protected against myocardial I/R injury, as demonstrated by the decrease in the percentage of infarct formation assessed by 2,3,5-triphenyl tetrazolium chloride (TTC) staining in representative heart tissue slices, comparing with the I/R control group. The levels of creatine kinase (CK), lactate dehydrogenase (LDH), malondialdehyde (MDA), and total superoxide dismutase (T-SOD) were tested. The ER stress-related proteins such as C/EBP homologous protein (CHOP), activating transcription factor 6 (ATF6), and (PKR)-like ER kinase (PERK) were further measured by western blot, and their messenger RNA levels were measured by real-time PCR. The apoptosis of heart tissue cells was also tested through the expressions of caspase-9, caspase-3, Bcl-2, and Bax proteins. Collectively, these results revealed that Sch B exerts protection role on myocardial I/R injury through decreasing oxidative reaction, suppressing ATF6 and PERK pathway, and attenuating ER stress-induced apoptosis.