We present a new method to study low-regularity problem for semi-linear dispersive equations without using X^{s, b} spaces. The main ingredient is a bilinear estimate which allows us to handle the high-low interactions in the nonlinearity. Moreover, this bilinear estimate is proved in a physical space approach by a new type of div-curl lemma introduced by Zhou. We will illustrate our method by reproducing optimal local well-posed results of Cauchy problem for modified KdV and modified BO equations. Further discussions will be also mentioned.