A P k -factor of complete bipartite graph K m,n is a spanning subgraph of K m,n such that every component is a path of length k. A P k -factorization of K m,n is a set of edge-disjoint P k -factors of K m,n which is a partition of the set of edges of K m,n . When k is an even number, the spectrum problem for a P k -factorization of K m,n has been completely solved. When k is an odd number, Ushio in 1993 proposed a conjecture. However, up to now we only know that Ushio Conjecture is true for k = 3. In this paper we will show that Ushio Conjecture is true when k = 5. That is, we shall prove that a necessary and sufficient condition for the existence of a P 5 -factorization of K m,n is (1) 3n 2m, (2) 3m 2n, (3) m + n ≡ 0 (mod 5), and (4) 5mn/[4(m + n)] is an integer.