Fermented soybean is one of the traditional foods widely consumed in many Asian countries. Most products (i.e., Natto, Jang, Kinema and Thua nao) are produced mainly by the bacterial activity. This study was performed as a part of the program to improve the soybean fermentation, and a use of the co-culture between the bacterium Bacillus subtilis and the fungus Rhizopus oligosporus was selected. For fermentation, the raw soybeans were washed, sterilized by autoclaving, and inoculated with a mixed microbial starter culture of B. subtilis (102 CFU ? g-1) and R. oligosporus (102 conidia ? g-1). The Free Amino Acid (FAA) contents were then investigated by using High Performance Liquid Chromatography (HPLC). The results showed that the amounts of FAAs and essential amino acids in the fermented soybean samples were found at significantly higher concentration when compared to those of the cooked, non-fermented soybean samples (p<0.05). These data indicated that an involvement of microbial fermentation helped to accelerate an increase of FAA. The food compositional data were useful as a reference (and/or guidance) for the nutritional value, when compared to other fermented soybeans and related products.