Although rheological properties are essential in scaling up fermenters, there has been no report on hydrogen fermentation of organic solid wastes. In the present work, hydrogen fermentation of food waste (FW) was conducted at various substrate concentrations (10-50 g Carbo. COD/L) and operational pHs (4.5-6.5), and rheological properties were attained. High hydrogen production performance showing over 1.7 mol H-2/mol hexose(added) was achieved at 10-40 g Carbo. COD/L and pH 5.5 and 6.0. The viscosity tended to increase with substrate concentration increase, and fermentation led to a significant reduction of viscosity. Before fermentation, zero viscosity and infinite viscosity of FW ranged 6.8-1274.1 mPa s and 2.2-58.1 mPa s, which were reduced to 10.4-346.2 mPa s and 1.1-5.3 mPa s after fermentation, respectively. Compared to substrate concentration, operational pH showed a less effect on rheological properties. At the similar volatile solids concentration range, hydrogen-fermented FW showed lower values of rheological properties than that of anaerobic digester sludge. Based on the agitation speed values assuring turbulent condition (complete mixing), the energy required for agitation can be reduced by 30-67% by lowering the applied agitation speed with fermentation time. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.