Abstract: PM_2.5 pollution raises environmental concern in China and other countries worldwide. The marker technique is regarded as a reliable tool for atmospheric aerosol source tracing. Cellulose can be used as a marker to identify and evaluate vegetative detritus emission to primary aerosols. An analytical method for cellulose in the PM_2.5 has been modified because of its low content in PM_2.5 and the relative high procedure blank. The method combines an enzymatic cellulose hydrolysis procedure and a GOD-phenol-4-aminoantipyrene determination of formed glucose. The key parameters of the method were optimized as a delignification condition, temperature for cellulose enzymatic hydrolysis, amount of enzyme, pH value and hydrolysis reaction time. The method meets the requirements of cellulose determination of PM_2.5 with a detection limit of 0.26 μg/m³. A procedure blank of 36.5 μg glucose was obtained which is significantly lower than 53.8 μg for the previous method. Cellulose in 23 Beijing urban PM_2.5 collected from May 15th to June 28th, 2012 were analyzed. The detection rate of cellulose is 96%. The results show that the cellulose concentration in Beijing PM_2.5 is (0.573±0.17) μg/m³. The vegetative detritus contribution is 1.37%±0.65% of PM_2.5 mass concentration, or 4.4% in average and 9.2% in the maximum of PM_2.5 organic carbon, indicating vegetative detritus is a major contributor of urban aerosol organic carbon and has to be considered in source attribution studies. This method provides a new technique for urban atmospheric PM_2.5 source identification.