Abstract: A method for preparing chloromethylphenylacetic acids represented by formula (II): wherein a methylphenylacetic acid represented by formula (I): is reacted with a chlorine gas, in an inert solvent, under the irradiation with light or in the presence of a radical initiator, is disclosed. According to the preparation method, high purity chloromethylphenylacetic acids can be prepared at a high yield, without using toxic sulfuryl chloride as a chlorinating agent, by chlorinating the methyl group of the methylphenylacetic acids at a high selectivity while suppressing by-production of a dichloro form or &agr;-chloro form.