In this paper, we study the isotropic-nematic phase transition for the nematic liquid crystal based on the Landau-de Gennes Q-tensor theory. We justify the limit from the Landau-de Gennes flow to a sharp interface model: in the isotropic region, Q is equivalent to 0; in the nematic region, the Q-tensor is constrained on the manifolds N with s+ a positive constant, and the evolution of alignment vector field n obeys the harmonic map heat flow, while the interface separating the isotropic and nematic regions evolves by the mean curvature flow. This problem can be viewed as a concrete but representative case of the Rubinstein-Sternberg-Keller problem introduced in Rubinstein et al. (SIAM J. Appl. Math. 49:116-133, 1989; SIAM J. Appl. Math. 49:1722-1733, 1989).