Ammonia-oxidizing archaea (AOA) and bacteria (AOB) play important roles in nitrogen cycling. However, the effects of environmental factors on the activity, abundance and diversity of AOA and AOB, and the relative contributions of these two groups to nitrification in paddy soils are not well explained. In this study, potential nitrification activity (PNA), abundance and diversity of amoA genes from 12 paddy soils in Southern China were determined by potential nitrification assay, quantitative PCR and cloning. The results showed that PNA was highly variable between paddy soils, ranging from 4.05±0.21 to 9.81±1.09 mg NOx-N kg-1 dry soil d-1 and no significant correlation with soil parameters was found. The abundance of AOA was predominant over AOB, indicating that AOA may be the major members in aerobic ammonia-oxidation in these paddy soils. Community compositions of AOA and AOB were highly variable among samples, but the variations were best explained by pH. AOA sequences were affiliated to the Nitrosopumilus cluster and Nitrososphaera cluster, and AOB were classified into the lineages of Nitrosospira and Nitrosomonas, with Nitrosospira being predominant over Nitrosomonas, accounting for 83.6% of the AOB community. Moreover, the majority of Nitrosomonas was determined in neutral soils. CCA analysis further demonstrated that AOA and AOB community structure were significantly affected by pH, soil total organic carbon, total nitrogen, and C/N ratio, suggesting that these factors exert strong effects on the distribution of AOB and AOA in paddy soils in Southern China. In conclusion, our results imply that soil pH was a key explanatory variable for both AOA and AOB community structure and nitrification activity.