It is shown numerically that for a polydisperse primary soot particles the soot temperature derived in two-color LII is biased towards the temperature of those larger and hotter particles. As smaller particles cool faster than larger ones, smaller particles gradually 'disappear', leading to a decrease soot volume fraction determined by the two-color LII. Based on numerically results, the relationship between LII signal and soot volume fraction can be summarized as: (1) It is linear in the two-color LII during and shortly after the laser pulse in the low-fluence regime (2) It is linear in the two-color LII only briefly around the peak of the laser pulse in the high-fluence regime (3) It is in general non-linear in the conventional LII method 5th international workshop on Laser-Induced Incandescence May 9-11, 2012, Palais des Congrès, Le Touquet, France