  The physics of a two-level system coupled to Ohmic bath is studied by means
of the perturbation approach based on a unitary transformation. Our main
results are: The coherence-incoherence transition is at $\alpha_c={1\over
2}[1+\Delta_r/\omega_c]$; for $\alpha<\alpha_c$ the dynamical quantity
$P(t)=\cos(\omega_0 t)\exp(-\gamma t)$; the sussceptibility
$\chi^{\prime\prime}(\omega)/\omega$ is of a double peak structure for
$\alpha<\alpha_c$ and the Shiba's relation is exactly satisfied; at the
transition point $\alpha=\alpha_c$ the real time correlation function
$C(t)\approx -1/\gamma^2_c t^2$ in the long time limit.
