  In this paper, we study the transition threshold of the 3D Couette flow in
Sobolev space at high Reynolds number $\text{Re}$. It was proved that if the
initial velocity $v_0$ satisfies $\|v_0-(y,0,0)\|_{H^2}\le c_0\text{Re}^{-1}$,
then the solution of the 3D Navier-Stokes equations is global in time and does
not transition away from the Couette flow. This result confirms the transition
threshold conjecture in physical literatures.
