We study the global-in-time dynamics of rarefied gas flow near global Maxwellian governed by the nonlinear Boltzmann equation in an infinite 3D layer $\Omega=\mathbb{R}^2\times (-1,1)$ supplemented with diffuse reflection boundary. It turns out that the solutions decay in time at a polynomial rate which is the same as that of solutions to the 2D heat equation along the tangent direction. Furthermore, using spectral analysis we characterizes the optimal large time asymptotic behavior of Boltzmann solutions in an infinite layer.