The density of aerosol particles was explored by using particles size measurement. A differential mobility analyzer (DMA) was used to select a monodisperse particle population with specific mobility diameter. The monodisperse particles beam enter aerosol time-of-flight mass spectrometer (ATOFMS). According to the time of flights between the two optical detection stages individual particle vacuum aerodynamic diameter was measured. Together with the mobility and aerodynamic diameters the quantitative relationship between them was yielded. Then the particle density for spherical, ρp, or particle effective density for aspherical, ρeff, was obtained. A calibration experiment was carried out by using dioctyl phthalate (DOP), with known chemical composition and density, on the system. Then the system was used to measure the particle effective densities of ammonium sulfate, sodium chloride, ammonium nitrate. The results are 1.211 g/cm3, 1.333 g/cm3, 1.039 g/cm3, respectively. The particle density for spherical olive oil is 0.914 g/cm3.