Work is devoted to research of influence of initial and current parameters of expansion sated vapor for the average size of the secondary particles formed in overcooling a stream. It is established, that at a heterogeneous way of condensation the basic influence on the size of particles of an aerosol is rendered with the reference temperature defining number of initial germs whereas speed of expansion of influence does not render. Numerical estimations of the size of secondary particles and degrees of condensation for equilibrium expansion carbon pair are lead.