Approximately 60 patients with ST-segment elevation myocardial infarction successfully treated with primary percutaneous coronary intervention will be enrolled and randomized to receive the sodium tanshinone IIA sulfonate in addition to standard therapy or the same volume/day of normal saline. The primary endpoint is the variation in LV end-diastolic volume index (LVEDVi) assessed with cardiac magnetic resonance imaging (MRI) at baseline and 6 months.