An improved method was proposed for the preparation of guanyl-o-methylisourea salts based on the addition reaction between dicyandiamide and methanol. Zinc salts have been found to serve as effective catalysts and furthermore, they convert the isourea formed into insoluble zinc complex salts. Guanyl-o-methylisourea zinc complex was given in a good yield, especially when zinc nitrate or zinc sulfate was used as a catalyst. Since the complex is readily decomposed into guanyl-o-methylisourea and zinc hydroxide upon treatment with hot water, the process is useful for the production of the isourea on a large scale. Some properties of those comoupnds were also discussed.