Methacrolein (MACR) and methyl vinyl ketone (MVK), which are the characteristic products of isoprene photooxidation in the atmosphere, as well as formaldehyde, were determined during March to November of 2006 in the ambient air of Beijing city. Cartridges coated with 2, 4-dinitrophenylhenylhydrazine (DNPH) were used to collect those carbonyls in ambient air and a high pressure liquid chromatography (HPLC) was used for quantification. Distinct amounts of MACR and MVK were detected during vegetable growing seasons from April to October with ambient levels in ranges of 0.11 x 10(-9) - 0.67 x 10(-9) and 0.19 x 10(-9) - 1.36 x 10(-9), respectively, with their maximal concentrations appearing in August. MACR and MVK were measured under the detecting limit in March and November, which may be ascribed to low isoprene emission from the broadleaf trees without leaves during two months. According to the stoichiometric coefficients among MVK, MACR, isoprene and formaldehyde, the concentrations of the reacted isoprene due to photo-oxidation can be calculated derived from the concentrations of the measured MVK and MACR. Then, the monthly (April to October) average concentrations of formaldehyde produced by the photo-oxidation of reacted isoprene were roughly estimated to be in the range of 0.35 x 10(-9) - 2.5 x 10(-9), which accounted for 4.6%-11.5% of total atmospheric formaldehyde. During the period with active atmospheric photo-oxidation (June to August), the contribution of biogenic isoprene to formaldehyde was predominant. Therefore, the biogenic isoprene is an important source to local photo-oxidant formation in Beijing and should be considered of.