Abstract: Laser Induced Incandescence (LII) is considered a very effective method for soot volume fraction measurement and can also be used to measure primary soot particle diameter with high spatial and temporal resolution. The laser fluence can potentially have a significant influence on the measured results. A high laser fluence in general results in a higher signal-to-noise ratio. But for soot particle size measurement, lower laser fluences should be used in order to avoid soot sublimation. To determine the optimal values of laser energy during the experiment, a combined approach of theoretical analysis and experiment was required. A mathematical model was established and analyzed firstly, then the primary particle size was measured using different laser fluences in the same position of a coflow ethylene/air laminar diffusion flame. Lastly, the optimal laser fluence was determined in the flame conditions investigated. This article laid the foundation for further investigations of the primary soot particle size measurement using low-fluence LII.