  Detection of relic gravitational waves, through their imprint in the cosmic
microwave background radiation, is one of the most important tasks for the
planned CMBPol mission. In the simplest viable theoretical models the
gravitational wave background is characterized by two parameters, the
tensor-to-scalar ratio $r$ and the tensor spectral index $n_t$. In this paper,
we analyze the potential joint constraints on these two parameters, $r$ and
$n_t$, using the potential observations of the CMBPol mission, which is
expected to detect the relic gravitational waves if $r\gtrsim0.001$. The
influence of the contaminations, including cosmic weak lensing, various
foreground emissions, and systematical errors, is discussed.
