The comprehensive campaigns were carried out at the urban site of Peking University, Beijing in the summer of 2011. The concentrations of organic aerosol (OA) and its mass spectra were measured by a high resolution time of flight aerosol mass spectrometers (HR-ToF-AMS). The sources of organic aerosol were investigated by combining organic ion fragment matrix with receptor models, positive matrix factorization (PMF). Four factors including low-volatility oxygenated organic aerosol (LV-OOA), semi-volatile organic aerosol (SV-OOA), cooking organic aerosol (COA), and hydrocarbon-like organic aerosol (HOA) were resolved out. A new source apportion method by combining spectra measured by AMS with chemical mass balance (AMS-CMB) was also established in this study. The AMS standard spectra in unit mass to charge ratio (m/z) from different sources were summarized by comparing spectra from lab and field studies. Five standard spectra were finally selected and input into CMB model to explore OA sources in Beijing summer. Good agreements of OA factors from AMS-CMB and AMS-PMF are observed. It is alsofound that concentrations of HOA factor from PMF model are equals to the sum of organic aerosol factors from biomass burning, coal burning and vehicle emissions in CMB model, revealing the advantage of AMSCMB method lie on the capability on primary sources apportion with low contributions (<5%). The final results by from CMB and PMF models show 66% of OA in Beijing summer are from secondary formations, following by cooking emissions (21%), biomass burning (6%), vehicle emissions (5%) and coal combustion(3%).