In vitro fermentation of 32 available feedstuffs was performed to investigate the relationship between soluble total organic carbon (TOCs) and volatile fatty acids (VFA) production. Correlation between VFA and methane (CH 4 ) emission was also investigated. For this purpose, a fermentation reactor was designed to collect liquor for estimating TOCs and VFA during in vitro fermentation. The results showed that the VFA production was proportional to TOCs produced during in vitro fermentation (y = 1.1169x, r 2 = 0.86). Forages produced more acetate (A) than propionate (P) and the A:P ratio of energy, protein and forages were 1.77:1, 2.08:1 and 2.80:1, respectively. CH 4 production of feeds was also found proportional to VFA produced during in vitro fermentation (y = 0.0508x, r 2 = 0.58). It might be stated from this in vitro study that TOCs production could be used as an indirect index for estimation of VFA of livestock feeds.