Cities are the most concentrated area of human habitation and activities, and their CO₂ emissions account for 71% of the world's total emissions. The study on the law of urban carbon emission has great significance for the reduction of the global carbon emission. In this study, we used the eddy covariance (EC) to measure CO₂ fluxes of a block in Beijing, China in 2015-2016, and analyzed the patterns of diurnal variations in CO₂ fluxes over different timescales. The diurnal variations in CO₂ fluxes exhibited (1) the obvious "bimodal" pattern:peaks of CO₂ fluxes appeared at 7:30-9:30 and 17:30-20:30 in the morning and the evening, respectively. (2) The weekend phenomona:Compared with weekday, the peak in the morning on weekends appeared about 1.5 hours later and 10.8% lower, while the peak in the evening was about 10.6% higher. (3) The seasonal dependence:The mean and daily peak values of CO₂ fluxes in winter were significantly higher than those in other seasons. There was an obvious valley in CO₂ fluxes at noon in summer. (4) Differences among wind direction:In the two main wind directions, the peaks varied greatly. (5) The weather influence:The "bimodal" pattern on cloudy days was more obvious than that on sunny days. Our research suggested that the diurnal variation of CO₂ fluxes in cities was mainly related to traffic volume, and was secondarily affected by vegetation. Therefore, the reduction in traffic emission and enhancement in vegetation sink play an important role in reducing carbon emissions in cities.