Abstract: Objective To investigate the correlation between microbiota-dependent metabolite trimethylamine N-oxide (TMAO) and cardiac function in old-old patients with chronic heart failure． Methods One hundred and four old-old patients (age≥80 years) with chronic heart failure were enrolled in the department of geriatrics, Nanjing General Hospital of Nanjing Military Area Command of Chinese PLA from January 2013 to February 2017, and they were divided into two groups: TMAO< 5μmol/L group and TMAO≥5μmol/L group． The basic clinical data, blood lipid levels, N-terminal pro-B-type natriuretic peptide (NT-proBNP) and left ventricular ejection fraction (LVEF) were analyzed and compared between two groups． Results Compared with TMAO< 5μmol/L group, the level of NT-proBNP in TMAO≥5μmol/L group increased significantly (5 603．00±770．40) ng/L vs (2 472．00±383．70) ng/L, P=0．019, while LVEF decreased significantly (35．00%±6．50% vs 42．00%±3．20%, P=0．024)． Plasma TMAO level was positively correlated with NT-proBNP (r=0．73, P< 0．01), and it was negatively correlated with LVEF (r=-0．58, P< 0．01)． Conclusion Plasma TMAO level is associated with cardiac function in old-old patients with chronic heart failure, which may be an important marker of cardiac function in old-old patients with heart failure．