A thermostable phosphatidylethanolamine N-methyltransferase and a process for producing the enzyme are provided. A host cell is transformed with an expression vector containing DNA encoding a thermostable enzyme derived from hyper-thermophilic archaea such as Pyrococcus, the enzyme having phosphatidylethanolamine N-methyltransferase activity and an optimum temperature of 90° C. or higher, and the transformed host cell is cultured.