, Adverb', one of the parts of speech in Russian, and Persian does not change in declensional cases. Syntactically, adverbs can have complements in Persian and Russian. Some linguists consider a comparative adverb the same as a changed adverb and they believe that adverbs change in Russian. In Persian, the syntactic function of a word in a sentence can distinguish an adjective from an adverb. It happens both in Persian and Russian that sometimes an adverb restricts a word or the meaning of a sentenct: to a specific sense. In Persian, all main components of a sentence can take complements. In neither of the languages, adverbs can ever come along with another word in genitive cases. They, however, develop a syntagmatic relation. A comparison of adverbs in both languages shows that some of the structures of the two languages, as far as the components of a sentence are concerned, are totally different from one another. Some impersonal sentences in Russian, in whose structure the role of a predicator is played by an adverb, are stated in personal sentences in Persian.