Abstract: In the period from 1999 to 2000, the N₂O vertical flux in the low atmosphere during the main growing seasons of rice was measured using conditional sampling method over a rice paddy at Changshu Agriculture Ecological Station. The measurements indicate that N₂O vertical flux was mainly between 0~2.0 mg·m^-2·h^-1, and the maximum daily averaged N₂O vertical flux was observed in the afternoon. The N₂O vertical flux in the transplant rice seedling period and reaping period was minimum among all the growing seasons. In these measurements, the minus N₂O vertical flux also observed in some cases. The N₂O vertical flux by conditional sampling technique was bigger than that measured by Chamber method reported in some domestic journal papers. The reason of this difference between the two methods needs further study.