What kind of action does the poetry writing bring children's life? And what kind of influence does it have on their whole development? These questions have need to be answered in establishing the systematic expansion of teaching poetry writing. They provide us the need of attention to not only the forms and techniques but also the internal operation of children when they write their poetry. James Britton, who is a scholar of language education in UK, created the developmental model of writing by re-apprehend the internal process of children behind writing, being inspired by language function theory of Roman Jakobson. The purpose of this article is clarifying how the act of poetry writing contributes to internal development of a child by examining Britton's developmental model and the theories behind it. As Britton showed, taking the view that a writer modifies his/her words, ideas, and emotions and renews his/her own evaluation system in organising and re-organising with any intentions gave suggestion to the issue what poetry writing is. In this article, I found the original elements which poetry writing has to development of a child's language expression by focusing on the Poetic function in three functions Britton presented. They admit to the educational significance of poetry writing that engender growth toward the writer that he/she keeps on re-organising himself/herself.