The present invention provides a process for preparing an ester comprising the step of reacting a carboxylic acid and an alcohol in the presence of an esterifying catalyst comprising at least one compound having the formula:O.dbd.M(RCOO).sub.2 wherein M represents Sn or Zn and R represents a saturated or unsaturated hydrocarbon group having 1-21 carbon atoms with or without side chains. According to the present process, substantially equal equivalents of the alcohol and the acid can be reacted completely. In addition, the present invention further provides a purification process to eliminate the catalyst from a homogeneous reaction system completely.