Ammonia-oxidizing microorganisms (AOMs) play important roles in nitrogen removal, but their metabolic activity in winter months in wastewater treatment plants (WWTPs) remains unclear. Here, two full-scale WWTPs were selected, and a total of 48 DNA-based stable isotope probing (DNA-SIP) microcosms of activated sludge samples at different winter months (November-January) and temperatures (13 degrees C, 25 degrees C and 37 degrees C) were constructed to evaluate the temporal heterogeneity and temperature response of active AOMs. The results provided direct evidence that the temporal heterogeneity of AOMs' activity was not obvious in winter months. In general, ammonia-oxidizing archaea (AOA) and bacteria (AOB) co-participated in the active ammonia oxidation in three winter months at in situ temperature (13 degrees C) in two WWTPs, and AOA were the dominant active AOMs, even though AOB outnumbered AOA in seed sludge. In contrast, higher peaks of AOA in "heavy" fractions at 25 degrees C and 37 degrees C also provided compelling evidence for the dominant contribution of AOA to active ammonia oxidation under higher temperatures. The Illumina sequencing of C-13-DNA obtained from the DNA-SIP microcosms at 13 degrees C in three winter months further suggest that Candidatus Nitrososphaera evergladensis and Candidatus Nitrosocosmicus exaquare were the dominant active AOA in two WWTPs, while the active AOB were dominated by Nitrosomonas oligotropha. At higher temperatures, active AOA shifted to Ca. N. evergladensis and Candidatus Nitrososphaera gargensis, and Nitrosomonas marina, Nitrosomonas europaea and Nitrosomonas communis dominated in the active AOB. Overall, AOA rather than AOB dominated the active ammonia oxidation in winter months and at higher temperatures in the full-scale WWTPs tested.