PURPOSE: To separate and to purify a saturated fatty acid by an ordinary separating means such as extraction, crystallization and distillation, by irradiating a mixture of saturated and unsaturated fatty acids with ultraviolet rays to convert the unsaturated fatty acid into a compound of different state.

CONSTITUTION: A mixture of saturated and unsaturated fatty acid (especially 8W24C, e.g. palmitic acid-palmitoleic acid) is irradiated with ultraviolet rays with 3W5mW/cm2 irradiation intensity for 30W60min so that the unsaturated fatty acid is converted into a compound of a different state and the saturated fatty acid is obtained by distillation. By this method, the saturated fatty acid can be readily separated from the mixture of the saturated and the unsaturated fatty acids having close boiling points and much higher safety is obtained than existing ozone oxidation method and halogenating method.