The paper analyzes the interjections and onomatopoeia as a special part of speech in Russian and Persian. Interjections may be a spontaneous bawl, an instinctive reaction to an external stimulus, which is peculiar not only for humans, but also animals. Interjections belong to a specific language and culture, require translation and special study. Interjection often serve not just as words, it often happens that one word means a sentence. Scientific studies have considered phonetic, structural, semantic and syntactic features of interjections and onomatopoeia, their classifications and their functional and communicative properties.