A significant amount of nitrous oxide ($N_2O$), which is one of the serious greenhouse gases, is emitted from nitrification and denitrification of wastewater. Batch wastewater nitrifications with enriched nitrifiers were carried out under oxygen-limited condition with synthetic (without organic carbon) and real wastewater (with organic carbon) in order to find out the effect of ammonium concentration on $N_2O$ emission. Cumulated $N_2O$-N emission reached 3.0, 5.7, 6.2, and 13.5 mg from 0.4 l of the synthetic wastewater with 50, 100, 200, and 500 mg/l ${NH_4}^+$-N, respectively, and 1.0 mg from the real wastewater with 125 mg/l ${NH_4}^+$-N. The results indicate that $N_2O$ emission increased with ammonium concentration and the load. The ammonium removal rate and nitrite concentration also increased $N_2O$ emission. Comparative analysis of $N_2O$ emission from synthetic and real wastewaters revealed that wastewater nitrification under oxygen-limited condition emitted more $N_2O$ than that of heterotrophic denitrification. Summarizing the results, it can be concluded that denitrification by autotrophic nitrifiers contributes significantly to the $N_2O$ emission from wastewater nitrification.