Crystalline form III of agomelatine, or N-[2-(7-methoxy-1-naphthyl)ethyl]acetamide is new. The compound is confirmed by powder X-ray diffraction pattern as given in the specification. An independent claim is included for preparation of the crystalline form III of agomelatine, or N-[2-(7-methoxy-1-naphthyl)ethyl]acetamide involves: heating agomelatine at 110[deg]C until fully melted, and then slowly cooled until recrystallization occurs. ACTIVITY : Hypnotic; Tranquilizer; Antidepressant; Gastrointestinal-Gen; CNS-Gen; Cardiovascular-Gen.; Neuroleptic; Muscular-Gen.; Immunomodulator; Eating-Disorder-Gen.; Anorectic; Anticonvulsant; Antidiabetic; Antiparkinsonian; Nootropic; Neuroprotective; Dermatological; Antimigraine; Cerebroprotective; Endocrine-Gen.; Cytostatic; Analgesic; Contraceptive. MECHANISM OF ACTION : Melatoninergic system receptor agonist; 5-HT(hydroxytryptamine)-2C receptor antagonist. No biological data given.本發明係關於一種由其粉末X射線繞射圖表表徵之式(I)之化合物的晶形III本發明亦係關於一種藥劑。