This paper presents a new theoretical analysis of the particle collection mechanism in the outer vortex of a cyclone. The radial component of the particle trajectory model is developed based on a force balance differential equation. The particle (with diameter of d₅₀) collection probability is 50% when it is under the force balance condition. The d₅₀ distribution in the space of the outer vortex is mathematically determined by solving the force balance differential equation. The particle collection probability distribution in the outer vortex is obtained based upon the inlet particle size distribution and the d₅₀ distribution. The critical separating diameter d₅₀ distribution and particle collection probability distribution in the outer vortex are the bases to simulate the collection efficiency and to predict the emission concentrations.