To provide a method for oxidizing an olefin, by which the oxidized product of the olefin at its double bond portion is obtained in a high yield, by oxidizing the olefin with hydrogen peroxide in the presence of a titanosilicate mesopore molecular sieve as a catalyst in the coexistence of a carboxylic acid.

This method for oxidizing an olefin comprises oxidizing (C) an olefin (for example, ethylene or cyclohexene) with (D) hydrogen peroxide (preferably a 20 to 90% aqueous solution) in the presence of (A) a titanosilicate mesopore molecular sieve as a catalyst preferably having a fine pore diameter of 1.5 to 10 nm in the mesopore region and a silicon/titanium ratio of 5 to 200 and in the coexistence of (B) a carboxylic acid (for example, formic acid or acetic acid) preferably at room temperature to 150°C. Therein, the component D is used in an amount of 0.1 to 10 moles per mole of the component C, and the component B is used in a volume of 0. 3 to 30 times the total volume of the components C and D. By the reaction, a vicinal diol compound and/or a vicinal diol monocarboxylate can be obtained in one step.