In this paper, we study the isotropic–nematic phase transition for the nematic liquid crystal based on the Landau–de Gennes $\mathbf{Q}$-tensor theory. We justify the limit from the Landau–de Gennes flow to a sharp interface model: in the isotropic region, $\mathbf{Q}\equiv 0$; in the nematic region, the $\mathbf{Q}$-tensor is constrained on the manifolds $\mathcal{N}=\{s_{+}(\mathbf{n}\otimes \mathbf{n}-\frac{1}{3}\mathbf{I}),\mathbf{n}\in {\mathbb{S}}^2\}$ with $s_{+}$ a positive constant, and the evolution of alignment vector field $\mathbf{n}$ 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).