There is provided a process for preparing a carboxylic acid ester of formula (3): R.sup.2COOR.sup.1 (3) wherein R.sup.1 is an alkyl group which may be substituted, an alkenyl group which may be substituted, an alkynyl group which may be substituted, an aralkyl group which may be substituted, or a heteroarylalkyl group which may be substituted, and R.sup.2 is an alkyl group which may be substituted, an alkenyl group which may be substituted, an alkynyl group which may be substituted, an aryl group which may be substituted, a heteroaryl which may be substituted, an aralkyl group which may be substituted, or a heteroarylalkyl group which may be substituted, which process is characterized by the steps of reacting a monohydroxy compound of formula (1): R.sup.1OH (1) wherein R.sup.1 is as defined above, with a zirconium compound of formula (6): Zr(OR.sup.8).sub.4 (6) wherein R.sup.8 is an alkyl group or an aryl group which may be substituted and is not the same as R.sup.1, to prepare a zirconium catalyst, and reacting a carboxylic acid of formula (2): R.sup.2COOH (2) wherein R.sup.2 is as defined above, with the monohydroxy compound of formula (1) in the presence of the zirconium catalyst.