Saturation curves for laser‐induced sodium fluorescence in a propane‐air flame under the cw and the pulsed excitations were measured to examine the contribution of chemical reaction via the excited states of sodium atom to saturated fluorescence. Saturation behavior for the pulsed excitation of ns duration is interpreted by the ordinary two‐level model taking account of spatial and temporal distribution of the laser intensity. The fluorescence intensity under the cw excitation levels off at much lower intensity than the ordinary saturation intensity. Consideration of species distributions in the flame indicates that the hydroxide formation process results in the saturation of the cw fluorescence. From the time‐resolved fluorescence measurement, the rate constant for this molecular formation is estimated to be around 1×107s−1.