  In this paper, we prove that if $u\in C([0,\infty),
\dot{H}^{1/2}_{a,1}(\mathbb{R}^3))$ is a global solution of 3D incompressible
Navier-Stokes equations, then $\|u\|_{\dot{H}^{1/2}_{a,1}}$ decays to zero as
time approaches infinity. Fourier analysis and standard techniques are used.
