The finemode aerosol concentration and the size distributions from the ground to 7 km over Beijing in 2004 are analyzed， by means of the data within and above the planetary boundary layer from the optical spectrometer probes mounted on a light aircraft. The results show that aerosol particles over the Beijing area had a uniform distribution below the mixed layer， decreased with height significantly between 1 to 2 km， and decreased slowly with height above 2 km. At higher levels， the number concentration of aerosols was smaller than 200 cm-3 with less variation. When there existed inversion， finemode aerosols often formed in the“accumulation layer”near the top of the boundary layer. Under highpressure conditions， the number concentration of aerosols near the ground was about 500 to 800 cm-3; while in a weak lowpressure case， the number concentration of aerosols near the ground was about 8000/cm3.The observed finemode aerosol size distributions show a singlepeaked mode.