The generation of aroma compounds and the changes in sensory characteristics through the soy sauce koji fermentation were investigated to evaluate the crucial impact of the koji fermentation on soy sauce aroma. The sensory analysis showed a marked increase in ‘musty’ and ‘soy sauce-like’ odours, whereas the beany attribute decreased significantly during koji fermentation. Aldehydes and alcohols were the major volatile classes present in the koji samples. Gas chromatography–olfactometry was used to identify the aroma-active compounds in harvested koji, and the results showed that 1-octen-3-ol, 3-octanol, (E)-2-octenal, di-epi-α-cedrene, benzeneacetaldehyde and 3-(methylthio)-propanal exhibited the greatest aroma intensities in harvest koji. Moreover, relative contents of (E)-2-octenal, benzeneacetaldehyde, 3-(methylthio)-propanal and 2-phenylpropenal were found to be positively related to ‘musty’ and ‘soy sauce-like’ odours. Results from principal component analysis showed that the growth and sporulation of Aspergillus oryzae had the important impacts on the volatile evolution.