Abstract. In this paper, we give some relationship between the nonlinearity of rational functions over F2n and the number of points of associated hyperelliptic curve. Using this, we get a lower bound on nonlinearity of rational-typed vector Boolean functions over F2n . While the previous works give us a lower bound on nonlinearity only for special-typed monomials, our result gives us general bound applicable for all rational fuctions defined over F2n . As an application of our results, we get a lower bound on nonlinearity of n × kn S-boxes.