Racemization of an optically active chrysanthemic acid derivatives of the formula: ##STR1## wherein X represents a chlorine atom, bromine atom or 2,2-dimethyl-3-isobutenyl-cyclopropane carboxyl group and * mark represents asymmetric carbon atom, is effected by contacting the acid derivatives with at least one compound selected from the group consisting of hydrogen bromide, a carboxylic acid bromide, an N-bromine compound, an S-bromine compound, a halobromine compound, a silicon-bromide compound, a phosphorus-bromide compound and an SH compound in the presence of a peroxide or an azo compound.