Vertical profiles of size-resolved particle concentrations were measured over the Tibetan Plateau by using balloon-borne optical particle counters. The measurement on 17 August 1999 showed that there was a thin layer of particles with diameters of >3.6 μm at concentrations of >0.01 cm−3 around 150 hPa. These particles are much larger than the background aerosols and most likely composed of ice crystals. Within the layer, a large number of submicron aerosols still coexisted with the ice particles, indicating that not all of the background aerosols were removed by ice nucleation and scavenging. Considering recent model and laboratory studies, these results suggest an occurrence of selective ice nucleation involving a fraction of the background aerosols (i.e., effective ice nuclei), which is associated with dynamical and constituent fields in the upper troposphere over the Tibetan Plateau.