  We investigate the target mass effects in QCD Bjorken sum rule. The magnitude
of the target mass correction is estimated in a variety of methods employing
positivity bound as well as the experimental data for the asymmetry parameters.
It turns out that the target mass correction is sizable at low $Q^2$ of the
order of a few GeV$^2$, where the QCD correction is significant. We show that
there exists uncertainty due to target mass effects in determining the QCD
effective coupling constant $\alpha_s(Q^2)$ from the Bjorken sum rule.
