Abstract: Homogeneous nucleation of supersaturated antimony vapor has been studied in a laminar-flow chamber with the use of transmission electron microscopy, diffusion spectrometer of aerosols, supersaturation cutoff, laser light scattering, and other methods. Surface tension σs of a critical nucleus and radius Rs of its surface of tension have been determined from the experimentally measured temperature field by numerical simulation of the processes occurring in the chamber. The calculation results are in good agreement with the measured profile of a deposit on chamber walls, the average size and concentration of formed aggregates, the average size of primary particles, and their number in an aggregate. Equivalent volume Veq = 0.87 cm3 of the zone of intense nucleation, average nucleation rate I = 5 × 1010 cm−3 s−1, average supersaturation S4 = 3.2 × 106, average temperature T = 421 K in this zone, and radius Rs = 0.63 nm and surface tension σs = 220 mN/m of critical nuclei formed in this zone have been calculated. The surface tension of antimony nuclei has appeared to be lower than the surface tension of its planar surface by 46%. In our previous works, it was found that the surface tension of critical bismuth nuclei was, on the contrary, markedly higher than that of its planar surface. Different signs of the deviation from the surface tension of a planar surface have, for the first time, been observed for elements of the same group.