This paper proposes a novel unconstrained criterion of blind equalization, which is based on minimizing the difference between two norms of the joint channel-equalizer impulse response. There is no need to impose any constraint on the equalizer coefficients when adjusting. The algorithm is convergent in the sense that a perfect equalization solution is achieved at every local optimum of the function. And we propose a batch algorithm and online-recursive method for implementation. Finally, the reliability, validity and good performance of the proposed algorithm are verified by evaluation of residual ISI through computer simulations.