BACKGROUNDSubstrate morphological change caused by microbial growth should affect oxygen transfer and hence solid-state fermentation (SSF) performance. Relationships between substrate morphological change caused by fungi growth and oxygen transfer in solid-state substrate were investigated based on porous media theory using fractal dimensions. RESULTSThe fungi biomass and oxygen diffusion rate in solid state fermentation followed the order 85%>75%>65% for moisture content experiments and 0.4 cm>1.5 cm>4.0 cm for particle length experiments, respectively, while permeability showed the opposite trends. Oxygen diffusion rate and permeability decreased as fermentation time was varied from 12 h to 48 h, and then increased with fermentation progress, which was consistent with the fractal dimension. Results indicated that fungi biomass, oxygen diffusion rate, and substrate permeability can be effectively expressed by a fractal dimension. CONCLUSIONCombining the kinetic model of permeability and fractal-oxygen exponential model analysis, oxygen transfer was closely related with substrate morphological change expressed by fractal dimension in solid-state fermentation of steam exploded rice straw using Penicillium decumbensJUA10. Thus, oxygen transfer expressed by a fractal dimension should be a more convenient way in solid-state fermentation. (c) 2014 The Authors. Journal of Chemical Technology & Biotechnology published by JohnWiley & Sons Ltd on behalf of Society of Chemical Industry.