Target Mass Effects in QCD Bjorken Sum Rule

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.