Objective To discuss the effect and mechanism of microRNA(miR)- 29b on the myocardial ischemia- reperfusion(IR)injury. Methods Healthy adult male Wistar rats were divided into four groups:sham group, IR group, miR- 29b group and scramble group.In sham group, 6-0 linea suturalis was placed without deligation, and in the rest three groups IR model was maded. Rats in miR- 29b group were given agomiR- 29b(1×103 mol/L)at 24 h before IR modelling, and those in scramble group were given agomiR- 29b(1 mmol/L) negative control sequence. Heart rate, left ventricular ejection fraction(LVEF),apoptosis rate of myocardial cells, collagen I and miR - 29b at 1,3,7,15 and 30 d after operation were observed. Results Heart rate in all groups showed no significant difference at 1,3,7,15 d,and 30 d.Compared with sham group,I/R rats showed decreased LVEF, as(70.42±2.06)%,(64.37±1.84)%,(54.61±1.37)%,(51.92±1.07)%, (47.38±1.02)% at 1, 3, 7, 15, 30 d, respectively. Compared with I/R group, LVEF in miR- 29b group were significantly increased,as(80.62±2.48)%,(78.37±2.03)%,(72.15±2.34)%,(65.18 ±1.73)%,(60.08±1.16)% at 1, 3, 7, 15,30 d, respectively. There was no significant difference between I/R group and scramble group. Compared with sham group, I/R rats showed increased apoptosis rates of myocardial cell, as(33.6±4.2)%,(53.7±6.3)%,(41.9±5.8)%,(39.3± 6.7)%,(32.7±4.8)% at 1, 3, 7, 15, 30 d, respectively. Compared with I/R group, apoptosis rates of myocardial cell in miR- 29b group were significantly decreased,as(18.2±3.9)%,(29.6± 5.2)%,(32.7±3.8)%,(28.4±6.0)%,(21.5±4.6)% at 1, 3,7, 15, 30 d, respectively. There was no significant difference between I/R group and scramble group.Compared with sham group, I/ R rats showed increased collagenⅠ. Compared with I/R group, collagenⅠin miR- 29b group were significantly decreased. There was no significant difference between I/R group and scramble group. Compared with sham group, I/R rats showed decreased miRNA- 29b mRNA.Compared with I/R group, miRNA- 29b mRNA in miR- 29b group were significantly increased. Conclusion The expression of miR- 29b was significantly decreased after myocardial IR, which indicated that miR-29b took apart in the process of myocardial IR injury. Up- regulation of miR- 29b could effectively ameliorate ventricular remodeling after myocardial IR injury and protect cardiac function. Key words: MicroRNA-29b; Myocardial ischemia; Reperfusion injury; Ventricular remodeling