The medium of solid-state fermentation (SSF) is a multiphase system, whose complex physical properties cause difficulties of the cognition of SSF process and the process control. This paper aimed to deeply analyze the mechanical, structural and thermal properties of solid-state medium, explored relationships between the medium's physical properties and fermentation performance, then to guide the process control and culture medium optimization for SSF. A mechanical property index (I-mp), which was the product of resilience, cohesiveness, and springiness, was established to fully characterize physical properties of medium. Results showed at the initial stage of SSF, there were positive correlations of I-mp with thermal conductivity and water retention, negative correlations with gas permeability and thermal diffusivity, and a parabolic relationship with biomass content with a symmetry-axis at I-mp = 4.37 x 10(-2). Correlations were further verified in SSF dynamic process. During the first 100 h, I-mp <= 4.37 x 10(-2) was positively correlated with heat conduction and water retention in medium, which was beneficial for cell growth; while when I-mp > 4.37 x 10(-2) during 100-168 h, medium heat accumulation, poor heat diffusivity and poor gas permeability indicated the reduction of fermentation performance. Results revealed medium physical properties significantly affected heat and mass transfer and further influenced fermentation performance in SSF. Mechanical property index I-mp could well comprehensively characterize physical properties of medium, which could be useful for guiding initial culture medium preparation and SSF process control. (C) 2017 Elsevier Ltd. All rights reserved.