The semi-continuous anaerobic co-digestion experiment was investigated with five treatments including (1) 100% rice straw (RS), (2) 100% water hyacinth (WH), (3) 100% pig manure (PM), (4) 50%RS : 50%PM, (5) 50%WH : 50%PM. Each treatment was conducted in triplicate with daily loading rate at 1 kgVS.m-3 and run for 60 days. The results showed that the components of volatile fatty acids (VFAs) were acetic acid, propionic acid, butyric acid, succinic acid, acrylic acid, fumaric acid, formic acid, malic acid. Acetic acid, propionic acid and butyric acid were the main component, accounting for approximately 80% of VFAs. The highest level of VFAs was recorded in 100%RS, 100%WH treatment at level of 33.1 and 39.6mmol.L-1, respectively. The concentration of long chain fatty acid such as propionic and butyric in 100% RS and 100% WH reactors were higher than other reactors. As a result, these treatments produced biogas more stable and higher than 100% pig manure (PM), 50%RS : 50%PM,  50%WH : 50%PM treatment. There was a strong relationship between VFAs and pH. Monitoring the components of volatile fatty acids in anaerobic process was highly recommended.