This paper is related about the concept of decycling set and decycling number of graph G. any subset of set of vertices F⊂V(G) is known as decycling set of graph G if the subgraph G-F is cycle free graph or acyclic graph. Then cardinality of smallest decycling set is known as decycling number of graph G and it is denoted by ∇(G). Here we will study the decycling number of the Cartesian product of a cycle graph (C_n) and path graph (P_2) ∀ n≥3. Graphs C_n□P_2 known as family of prism graphs and it is denoted by Y_n. Moreover, we will find a vertex set F_n whose deletion in prism graphs (Y_n) gives acyclic graph. This paper also proves that the decycling number of prism graph is