In this paper, the effect of the wick layer thickness on the thermal performance of thin flat plate heat spreaders under the fixed height less than 1mm with multi-sources and sinks are theoretically presented using analytical solutions for 2D temperature distribution and pressure distribution. Especially, we show whether the optimized wick layer thickness in the heat spreaders is for maximum heat transfer rate with both capillary limit and allowable maximum temperature under the fixed conditions. Based on the results, optimized wick layer thickness was theoretically presented. The results can help thermal engineer to design the thin flat plate heat spreader with multi-sources and sinks.