Ozone (O3) is a primary gaseous contaminant in urban ambient, and its variation correlates well with nitrogen oxides (NO(x) = NO+ NO2). Continuous monitoring in the urban area from August 2004 to July 2005 was carried out in Beijing, using 49C ozone analyzer and 42CTL NO-NO2-NO(x) gas analyzer. The results show that peak O3 and oxidant (OX = O3 + NO2) concentrations in Beijing often appear at about 15:00 in the early afternoon, the diurnal variation of NO, concentrations show a very clear cycle with two peaks, one appears at about 07:00 in the morning and the other is at 23:00 in the evening. However, concentrations variation in four seasons are not same. The highest concentrations of O3 and NO(x) appear in summer and winter, respectively. The NO, crossover point occurs at about 100 x 10(-9) volume ratio. At lower level, NO2 is the major component of NO(x), whereas NO dominates at higher mixing ratio. It is also shown that the level of OX is made up of two contributions, regional contribution and local contribution. The former is affected by regional background O3 level, with the maximum in spring, whereas the latter is effectively correlates with the level of primary pollution, with the maximum in summer. Diurnal variation also appears in the concentrations of the components of oxidants.