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The greenhouse gas (GHG) [methane (CH4) and nitrous oxide (N2O)] mitigation effects of mixing dried grass into passively-aerated manure during the composting process (which accounts for 68.7% of Japanese dairy manure management) were assessed. Gaseous emissions [CH4, N2O, carbon dioxide (CO2) and ammonia (NH3)] from about 4?t of fresh dairy manure with or without 400?kg of dried grass mixed in were measured by the dynamic chamber method. The addition of dried grass contributed to a decrease in GHG emissions from 20.8?±?1.3?g?kg?1 volatile solids (VS) to 5.4?±?1.4?g?kg?1 VS (74.3% mitigation) for CH4 and from 7.4?±?2.6?g N2O-N kg?1 Ninitial to 2.7?±?0.4?g N2O-N kg?1 Ninitial (62.8% mitigation) for N2O. By applying this strategy, the expected reduction of GHG emission would be 70,466?t CH4 yr?1 and 1379?t N2O-N yr?1 (1907?Gg CO2 eq. yr?1 in total) in the Japanese dairy sector. On the other hand, it was showed that CO2 and NH3 emissions increase [from 424.4?±?214.9?g CO2 kg?1 VS to 603.8?±?99.6?g CO2 kg?1 VS for CO2 and from 16.9?±?7.1?g ammonium-nitrogen (NH3-N) kg?1 Ninitial to 38.3?±?3.5?g NH3-N kg?1 Ninitial for NH3] by this method. Moreover, the mechanism of this significant N2O mitigation effect cannot be explained, and a better understanding of this effect could further improve the GHG mitigation strategy.