In this paper, we investigate the Cauchy problem for the Ostrovsky equationin the Sobolev space H −3/4 (R). Here β > 0(< 0) corresponds to the positive (negative) dispersion of the media, respectively. P. Isaza and J. Mejía (2006) [13], (2009) [15], K. Tsugawa (2009) [26] proved that the problem is locally well-posed in H s (R) when s > −3/4 and ill-posed when s < −3/4. By using some modified Bourgain spaces, we prove that the problem is locally well-posed in H −3/4 (R) with β < 0 and γ > 0. The new ingredient that we introduce in this paper is Lemmas 2.1-2.6.