This study proposed a novel free ammonia (FA, i.e., NH3) pretreatment technology to enhance anaerobic methane production from primary sludge for the first time. The solubilization of primary sludge was substantially enhanced following 24h FA pretreatment (250-680mg NH3-N/L), by which the release of soluble chemical oxygen demand (SCOD) (i.e., 0.4mg SCOD/mg VS added; VS: volatile solids) was approximately 10 times as much as that without pretreatment (i.e., 0.03mg SCOD/mg VS added). Then, biochemical methane potential (BMP) tests demonstrated that FA pretreatment of 250-680mg NH3-N/L was capable of enhancing anaerobic methane production while the digestion time was more than 7 days. Model based analysis indicated that the improved anaerobic methane production was due to an increased biochemical methane potential (B-0) of 8-17% (i.e., from 331 to 357-387L CH4/kg VS added), with the highest B-0 achieved at 420mg NH3-N/L pretreatment. However, FA pretreatment of 250-680mg NH3-N/L decreased hydrolysis rate (k) by 24-38% compared with control (i.e., from 0.29d(-1) to 0.18-0.22d(-1)), which explained the lower methane production over the first 7 days' digestion period. Economic analysis and environmental evaluation demonstrated that FA pretreatment technology was environmentally friendly and economically favorable. Biotechnol. Bioeng. 2017;114: 2245-2252. (c) 2017 Wiley Periodicals, Inc.