Effects of coprophagous insects on greenhouse gas emissions from cattle dung pats were investigated during the initial stage in the decomposition of dung, with accompanying changes in nitrogen, carbon, and energy content. We set up three treatments with adults of Caccobius jessoensis Harold (dung beetle) and larvae of the fly Neomyia cornicina (F.): 1) dung with dung beetles; 2) dung with fly larvae; and 3) dung without insects. In these treatments, the gas flux was measured from air flow exiting the glass containers connected with an in vitro continuous gas analysis system. Total gas fluxes from dung pats with fly larvae were lowest in carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The presence of dung beetles significantly increased CO2 flux from dung, but reduced CH4 flux compared with dung without insects. Fluxes of N2O from dung pats with dung beetles and without insects had distinct peaks at different times after the start of the experiment, while N2O from dung with fly larvae was emitted in extremely low levels throughout the experiment. Carbon (C) content in dung with beetles was significantly lower than that of untreated dung pats designated as fresh dung, whereas that of dung with fly larvae was higher than dung with beetles and without insects. Nitrogen (N) content was significantly lower in dung with fly larvae than the other treatments. Contents of C and N in fly pupae were 35.87 and 8.05%, respectively. During the larval growth of the fly, energy accumulated in the fly body was 2,830 J/g.