We consider an RSA variant with Modulus N = prq.This variant is known as Prime Power RSA. In PKC 2004, May proved when decryption exponent d < N r/(r+1)2 or d < N (r-1/r+1)2, one can factor N in polynomial time. In this paper, we improve this bound when r ≤ 5. We provide detailed experimental results to justify our claim. © 2014 Springer Science+Business Media New York.