Abstract： Objective To investigate the effect of intramyocardial injection of slow release microspheres of tannic acid （TA） on ventricular remodeling after acute myocardial infarction （AMI） in rats. Methods Slow release microspheres of TA were prepared and the release parameters in vitro were detected. AMI model in rats was induced. Eighty rats were enrolled and divided into 4 groups by random digital table：poly （lactic-co-glycolic acid） （PLGA） microspheres injection （PLGA group， n=24）， PLGA-TA microspheres injection （PLGA-TA group， n=24）， TA injection group （TA group， n=16） and normal saline injection group （NS group， n=16）. Heart function was evaluated by echocardiography after the injection. The structure of cardiac extracellular matrix （ECM） in the infarcted borderline area was evaluated at 4th week after the injection. Collagen content in the infarcted area was evaluated by hydroxyproline colorimetry assay at 2nd and 4th week after the injection. Results TA release was maintained at a constant rate from the microspheres for one month in vitro. Two weeks after the injection， left ventricular ejection fraction（LVEF）， left ventricular fraction shortening（LVFS）， left ventricular end-diastolic diameter（LVEDD） and left ventricular end-systolic diameter（LVESD） in PLGA-TA group and TA group were significantly better than those in the other two groups（P＜0.05）. Four weeks after the injection， LVEF， LVFS， LVEDD and LVESD in PLGA-TA group were significantly better than those in the other three groups （P＜0.05）. Four weeks after the injection， slow release microspheres of TA in the PLGA-TA group effectively improved the arrangement of ECM compared with TA group. Four weeks after the injection， collagen content in the infarcted area of PLGA-TA group was significantly higher than that in TA group（88.88±7.28 μg/mg dry weight vs. 72.43±9.02 μg/mg dry weight）， PLGA group（88.88±7.28 μg/mg drg weight vs. 71.97±6.06 μg/mg dry weight） and NS group（88.88±7.28 μg/mg dry weight vs. 68.86±7.55 μg/mg dry weight， F=7.162，P=0.003）， but there was no statistical difference in the collagen content of the infarcted area among TA group， PLGA group and NS group （P＞0.05） . Conclusion Intramyocardial injection of slow release microspheres of TA can maintain a constant release of TA for a comparatively long period， inhibit collagen matrix degradation， and effectively attenuate ventricular remodeling after AMI in rats.