Average Kinetic Energy of Heavy Quark and Virial Theorem

We derive the virial theorem of the relativistic two-body system for the study of the B-meson physics. It is also shown that the solution of the variational equation always satisfies the virial theorem. From the virial theorem we also obtained $\mu_\pi^2 \equiv -\lambda_1 \equiv < p^2> = 0.40\sim 0.58$ GeV$^2$, which is consistent with the result of the QCD sum rule calculations of Ball $et$ $al.$