A mechanistic model for K-containing vapors transforming into sub-micrometer particles in the flue gas postpulverized straw combustion was developed based on a plug-flow reactor model. The model considers the mechanisms including KCl sulfation, homogeneous nucleation of the vapors, heterogeneous condensation of the vapors on existing particles, and collision-coagulation between aerosol particles to describe the transformation processes, with a detailed view of the effects of the associated interactions. The behaviors of pure KCl transforming into sub-micrometer particles, the impact of KCl sulfation on KCl transformation, and the influence of fly ash particles on the transformation of K-containing vapors to sub micrometer particles were investigated numerically by model simulation. The results show that homogeneous nucleation of KCl starts at 953-983 K, and the initial concentration of KCl vapor is the dominant factor determining the nucleation temperature and size. However, gaseous K2SO4 generated by KCl sulfation begins to nucleate homogeneously at 993-1093 K, and the resulting K2SO4 particles prevent KCl vapor from nucleation but allows it to condense on existing particles. The residual fly ash has a significant effect on the vapor transforming to sub-micrometer particles. It not only reduces the mass concentration and size of the vapor-formed sub-micrometer particles but also the number concentration of the sub-micrometer ash.