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.