In this paper, we mainly prove a congruence conjecture of Z.-W. Sun involving Franel numbers: For any prime p > 3, we have Sigma(p-1)(k=0)(-1)(k) f(k) equivalent to (p/3) + 2p(2)/3B(p-2) (1/3) (mod p(3)), where B-n(x) is the n-th Bernoulli polynomial.