Fine urban aerosol particles with aerodynamic equivalent dimeter ≤2.5 μm (PM2.5) were collected in Mainz (a city within the Rhine-Main area, the third largest metropolitan region in Germany) and Beijing (Chinese megacity). A solvent mixture of acetonitrile-water was used to extract the organic aerosol fraction (OA) from the particle samples. The extracts were analyzed by an ultrahigh resolution mass spectrometer (UHRMS) Orbitrap coupled with ultra-high-performance liquid chromatography (UHPLC) both in the negative and positive ion mode. The number of compounds observed in Beijing is a factor of 2–10 higher compared to Mainz. The clear differences on chemical composition of OA in the two cities were observed. The majority of organics in Beijing OA is characterized by lower elemental H/C and O/C ratio but a higher degree of unsaturation and a larger aromaticity equivalent (XC) compared to Mainz OA, suggesting that aromatics, which are related to direct combustion compounds (e.g., oxidized polycyclic aromatic hydrocarbon (PAH)), play an important role for OA in Beijing. A significant number of organosulfates (OSs) with long-carbon chain and low degree of unsaturation were observed in Beijing OA, indicating that long-chain alkanes emitted by vehicle might be their precursors.