In a method of manufacturing fatty acid esters of polyoxyalkylene alkyl ethers, fatty acid alkyl ester is reacted with alkylene oxide in the presence of a catalyst consisting essentially of a magnesium oxide added with at least one metal ion selected from the group consisting of Al.sup.3+, Ga.sup.3+, In.sup.3+, Tl.sup.3+, Co.sup.3+, Sc.sup.3+, La.sup.3+, and Mn.sup.2+ to manufacture a compound having the following formula: ##STR1## wherein each of R.sub.1 and R.sub.3 independently represents an alkyl group, R.sub.2 represents an alkylene group, and n is a positive integer.