Abstract: A 1-cyclohexyl-1-methylethyl peroxy ester, a novel compound, represented by the formula: ##STR1## wherein R.sup.1 stands for one member selected from the group consisting of H and alkyl groups and R.sup.2 and R.sup.3 independently stand for an alkyl group, provided that R.sup.2 and R.sup.3 each stand for an alkyl group of 1 to 5 carbon atoms and the sum of the carbon atoms of R.sup.2 and R.sup.3 is in the range of from 2 to 6 where R.sup.1 is H and R.sup.1, R.sup.2, and R.sup.3 each stand for an alkyl group of 1 to 9 carbon atoms and the sum of the carbon atoms of R.sup.1, R.sup.2, and R.sup.3 is in the range of from 3 to 11 where R.sup.1 is an alkyl group, a polymerization initiator for a vinyl group-containing compound monomer and a curing agent for an unsaturated polyester resin, which polymerization initiator and curing agent both contain as an active component thereof the ester mentioned above.