The morbidity and mortality of heart failure with preserved left ventricular ejection fraction (HFpEF) were similar to those of systolic heart failure, but the pathogenesis of HFpEF remains poorly understood. It was demonstrated that, in systolic heart failure, microRNA-21 (miR-21) could inhibit the apoptosis of cardiac fibroblasts, leading to cardiac hypertrophy and myocardial fibrosis, but the role of miR-21 in HFpEF remains unknown. By employing cell culture technique, rat myocardiocytes and cardiac fibroblasts were obtained. The expression of miR-21 in the two cell types under different conditions was compared and we found that the miR-21 expression was significantly higher in cardiac fibroblasts than in myocardiocytes. We established a rat HFpEF model and harvested the tissues of cardiac apex for pathological examination, Northern blotting and so forth. We found that miR-21 expression was significantly higher in model rats than in sham-operated rats, and the model rats developed the cardiac atrophy and cardiac fibrosis. After injection of miR-21 antagonist, the the cardiac atrophy and cardiac fibrosis were conspicuously ameliorated. Both in vivo and in vitro, inhibition of miR-21 expression resulted in reduced Bcl-2 expression while over-expression of miR-21 led to elevation of Bcl-2 expression. Our study suggested that miR-21 promoted the development of HFpEF by up-regulating the expression of anti-apoptotic gene Bcl-2 and thereby suppressing the apoptosis of cardiac fibrosis.