RP-3 is the most popular aviation fuel in China and its soot emission raises serious concern. The current study analyzed the soot precursors of RP-3 and n-dodecane in laminar flame, and integrated the corresponding poly-aromatic hydrocarbons (PAHs) growth mechanisms with popular RP-3 and n-dodecane oxidation mechanisms to improve the soot formation prediction for both fuels. The sizes of soot precursors were determined by the fringe lengths of soot particles. At dimensionless height DH = 0.25 of RP-3 and DH = 0.50 of n-dodecane, the geometric mean fringe lengths of RP-3 and n-dodecane are measured to be 0.67 nm and 0.64 nm respectively, which approximate to the size of the five-ringed (A5) poly-aromatic hydrocarbon. An A5 growth mechanism was then added on both popular RP-3 and n-dodecane oxidation mechanisms respectively. The base oxidation mechanisms for both fuels only contain the mono-aromatic hydrocarbons formation and benzene was treated as soot precursors in simulation. After validation by the ignition delay time and flame speed in literature, the combined mechanisms were then used to simulate the soot property and validated by the measured primary particle diameter. The simulation results suggested that the computed soot precursor concentrations of the combined mechanisms are smaller than that of the base mechanism. The combined mechanisms enhance the soot nucleation process which increases the primary particle number density prediction, and weaken the soot growth process which decreases the soot volume fraction prediction for both fuels. Eventually, the proposed combined mechanisms including the large PAHs growth provide a better agreement with the measured primary particle diameter.