This paper deals with the poetic necessity of Qudamah Ben Jaafar, he begins by explaining the term necessary, and then he displays the emergence of Arabic grammar and its motives; he also refers to the Arabic criticism which was subjective without depending on standards, until the beginning of blogging era and development of science; it has being influenced by other cultures, particularly Greek philosophy. The most affected person, put foundations and controls for criticism he was Qudamah Ben Jaafar, who wrote a book called (criticism of poetry) which pointed to the failure of his predecessors who launched judgments on the poetry and limited their attention to the rhyme, strange and meanings, and neglected the difference between good and bad poetry, he put limits to the poetry, and clarified its pillars, and explained the reasons of the good and bad quality of poetry, and judged defects on the poems in which their content are disturbed (pronunciation, weight, rhyme, meaning), and these defects included damages and so on.