In this paper we consider the fundamental production inventory problem such that the product quantity is a triangular fuzzy number Q = (q 1 ,q 0 ,q 2 ), where q 1 = q 0 = Δ 1 , q 2 = q 0 + Δ 2 . Suppose q * denotes the crisp economic product quantity in the classical production inventory model and we assume 0 < q 1 < q * < q 0 < q 2 or 0 < q 1 < q 0 < q * < q 2 . According to two relations of q * and q 1 ,q 0 ,q 2 (q 1 < q 0 < q 2 ) we find the membership function μ F ( Q ) (y) of the fuzzy cost function F(Q) and their centroid, then obtain the economic product quantity q * * in the fuzzy sense.