It was found that hydrogen plays a most important role as an acceleration factor of acetate decomposition to methane; this was investigated based on the theory of catabolized reactions of acetate, propionate, and butyrate. To demonstrate the hydrogen effect, several batch experiments were carried out with the fixed biofilm methane fermentation reactor(FBMFR) using a sole substrate of acetate, propionate and butyrate or a mixture substrate(acetate-propionate,acetate-butyrate). While using each volatile fatty acid and the mixture, the experiment of continuous incubation was carried out to investigate biofilm biota and structure through a scanning electron microscope. From these experimental results, the authors present a new reaction rate equation of acetate decomposition.