In this paper, we prove that a finite group G is isomorphic to the alternating group An if and only if the normalizers of their p-Sylow subgroups have the same order for every prime p. Moreover, if n ≥ 4 with n ≠ 8,10 and r is the greatest prime not exceeding n, then a finite group G is isomorphic to An if and only if |G| = |An| and the normalizers of their r-Sylow subgroups have the same order.