The protective effects of Sini decoction (SD) on experimental myocardial ischemia were studied. Results indicated that index for glycogen granules of ischemic myocardium in SD group was higher than that of control group (11.3+I.8 vs5.8+1.1, P<0.01), suggesting SD decreases the ischemia-induced myocardial glycogen consumption. In SD group, mitochondrial specific surface of myocardium was larger (6.33+0.17 vs 5.35+0.26, P<0.01), and mitochondrial, volum density of myocardium lower (33.45+2.71vs 37.65+1.83 96, P<0.05) than those of the ischemic group, suggesting that SD significantly reduced the ischemia-induced mitochondrial swelling. There was no difference in the myofibril volume density between the two groups (P>0.05). It indicated that SD could significantly prevent ischemic injury to myocardium.