Coconut inflorescence sap (CIS) is sweet, oyster-white and translucent and was reported to be highly nutritive and a good digestive agent. The chemical composition changes including total sugar, reducing sugar, ethanol, total acidity, volatile acid, amino acid, vitamin C and total phenolic contents of postharvest coconut inflorescence sap (PCIS) were investigated during a 12-day natural fermentation, and the variety and content of phenolic compounds of fresh coconut inflorescence sap (FCIS) and natural fermented coconut inflorescence sap (NCIS) were also studied by an high performance liquid chromatography (HPLC) system. Total acid, volatile acid and total phenolic contents increased during natural fermentation, while total sugar contents decreased during natural fermentation. The amino acid content declined steadily after harvesting until 3 days, but then remained almost constant. Vitamin C content decreased on day 1, slowly rose to 20.7 mg/L on day 3, and then decreased obviously. Five kinds of phenolic compounds were detected by HPLC. These compounds all increased in NCIS compared with FCIS; both NCIS and FCIS also contained other kinds of phenolic compounds.Key words: Coconut inflorescence sap (CIS), post-harvest, natural fermentation, chemical composition.