By using eddy covariance technique, this paper measured the net ecosystem CO2 exchange (NEE) in a reed (Phragmites australis) wetland in the Yellow River Delta of China during the growth season of 2011, and investigated the variation patterns of the NEE and related affecting factors. The average diurnal variation of the NEE in different months showed a U-type curve, with the maximum net CO2 uptake rate and release rate being (0.44 +/- 0.03) and (0.16 +/- 0.01) mg CO2 x m(-2) x s(-1), respectively. The NEE, ecosystem respiration (R(eco)), and gross primary productivity (GPP) were all higher in vigorous growth season (from July to September) and lower in early growth season (from May to June) and late growth season (from October to November). Both R(eco) and NEE reached their maximum values in August, while GPP reached its peak value in July. During the growth season, the ecosystem CO2 exchange was mainly dominated by photosynthetic active radiation (PAR), soil temperature (T(s)), and soil water content (SWC). There was a rectangular hyperbolic relationship between the daytime NEE and PAR. The nighttime ecosystem respiration (R(eco,n)) was exponentially correlated with the T(s) at 5 cm depth, and the temperature sensitivity of the ecosystem respiration (Q10) was 2.30. SWC and T(s) were the main factors affecting the R(eco,n). During the entire growth season, the reed wetland ecosystem in the Yellow River delta was an obvious carbon sink, with the total net carbon sequestration being 780.95 g CO2 x m(-2).