In this study biogas production efficiency was evaluated by adding ultra-fine bubble water (UFBW) into waste activated sludge (WAS) through anaerobic digestion (AD). Four kinds of gases, i.e. Air, N-2, CO2 and H-2 were introduced into tap water (TW) to prepare the UFBW with their properties being first investigated. Results show that hundreds of millions of nanoscale bubbles with the negative zeta potential could be stable in the UFBW for longer than two weeks whereas almost no nanometer bubbles could be detected in the raw TW. As for their impact on subsequent AD of WAS, the cumulative methane production with the addition of UFBW was 14-21% higher than that from the raw TW addition group. Interestingly, the Air-UFBW also could promote the biogas production in this study, which is different from the common understanding of AD, an obligate anaerobic process.