Effects of the freeze thaw process on physical properties, cell viability, microbial activities and population structures of anaerobic sludge were investigated. It was found that the sludge volume index was greatly reduced from 16.4 mL/g in the original sludge to 4.0 mL/g in the solid fraction of the frozen thawed sludge. Even though the freeze thaw process decreased cell viability in the solid fraction of the frozen thawed sludge, microbial activity tests showed that the freeze thaw process enhanced acidogenic activity approximately 20%. The enhanced acidogenic activity of the solid fraction was in good agreement with the enrichment of Clostridiaceae, Porphyromonadaceae and Propionibacteriaceae found in the solid fraction. The relative abundances of Proteobacteria families Oxalobacteraceae, Moraxellaceae, and Pseudomonadaceae were found to be highest in the liquid fraction where they form a substantial proportion of the bacterial community (a total of 59%). (C) 2016, The Society for Biotechnology, Japan. All rights reserved.