The benzoxazine derivatives are a new chemical family of diphenyl ether herbicides, which exhibit a strong peroxidizing herbicidal activity on mono- and dicotyledonous species in preemergence and postemergence tests. Twenty derivatives were synthesized, and their herbicidal activity was determined to examine structure-activity relationships. Among the compounds investigated, it was found that the fluorine atom introduction into the trifluoromethylbenzene moiety together with an oxazine ring instead of a nitro group led to the most active herbicide.