Abstract: The amount of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in quaternary red-clay paddy soils during rice growing season were determined by real-time polymerase chain reaction (RT-PCR) using amoA gene assay copies. The potential nitrification rate of soil was investigated too. Results show that AOA is the dominant ammonia-oxidizer in quaternary red-clay paddy soils, with AOA/AOB amoA gene ratio range of 1.6~120.7. There is a higher abundance of AOA in root-zone soils than in surface soils. The amount of AOA in surface and root-zone soils increases with growth stage of rice, which is more evident in root-zone soils. There is a higher abundance of AOB in surface soils than in root-zone soils at the early growth stage, with obvious enhancement of root-zone AOB after booting stage. Root-zone AOB is much abundant at the late growth stage of rice. Potential nitrification rate gradually increases with time during rice growing season, especially in root-zone soils. Potential nitrification rates in root-zone soils exhibit significant positive correlations with AOB and AOA amounts. However, potential nitrification rates in surface soils are only significantly positively correlated with AOA amount. The findings of this study indicate the dominance of AOA, which is more strongly related with quaternary red-clay paddy soil nitrification. The results further demonstrate the importance of ammonia-oxidizers in microbial composition and ecological functions of paddy soils.