This study investigated the hydrogen fermentation from galactose in up-flow anaerobic sludge blanket (UASB) reactor under mesophilic temperature of 37 degrees C. The maximum hydrogen production rate (HPR) and hydrogen yield (HY) of 56.8 L/L/d and 2.25 mol/mol galactose added were achieved at a hydraulic retention time (HRT) of 2 h. Further shortening the HRT to 1.5 h, led to the significant drop in the HPR and HY with a value of 48.3 L/L/d and 1.44 mol/mol galactose added, respectively. During the reactor operation, neither external pH adjustment nor recirculation was required, while the pH was maintained in the range of 5.5-6.2 by the carbonate buffer in the nutrient medium. Acetate and butyrate were the major soluble metabolic products (SMPs) formed during the hydrogen fermentation with fewer amounts of lactate and propionate during the peak hydrogen production performances. The results showed that controlling appropriate HRT is prerequisite for the enhancement of hydrogen production performances. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.