Objective To investigate the efficacy of coenzyme Q10(CoQ10) in patients with chronic cardiac dysfunction. Methods One hundred cases with New York Heart Association (NYHA) Class Ⅱ or Ⅲ heart failure were enrolled in this double-blind, placebo-controlled study. Patients were randomly divided into CoQ10 group (n＝50) and control group (n＝50), and were administrated with 100 mg/day of oral CoQ10 or placebo respectively for 3 months. Results All patients completed the trial. After 3 months of treatment, NYHA class in CoQ10 group showed a significant improvement compared with control group (P＝0.01). There was a significant (P＝0.045) increase of the 6-min walk-test distance in CoQ10 group but no change in control group (P＝0.63). A tendency of increase of left ventricular ejection fraction was observed in CoQ10 group (P＝0.059). There was a threefold increase of CoQ10 level in CoQ10 group [(0.70±0.06) vs (2.15±0.26) mg/L], but no change was observed in control group. There was a significant decrease in the serum brain natriuretic peptide level in CoQ10 group (P＝0.002), but not in control group (P＝0.31). Conclusions This pilot study suggests that CoQ10 therapy improves cardiac function in patients with NYHA class Ⅱ or Ⅲ heart failure.