  GRB 120308A, a long duration $\gamma-$ray burst detected by {\it Swift}, was
distinguished by a highly-polarized early optical afterglow emission that
strongly suggests an ordered magnetic field component in the emitting region.
In this work we model the optical and X-ray emission in the reverse and forward
shock scenario and show that the strength of the magnetic field in reverse
shock region is $\sim 10$ times stronger than that in the forward shock region.
Consequently the outflow powering the highly-polarized reverse shock optical
emission was mildly-magnetized at a degree $\sigma \sim$ a few percent.
Considering the plausible magnetic energy dissipation in both the acceleration
and prompt emission phases of the Gamma-ray Burst (GRB) outflow, the afterglow
data of GRB 120308A provides us the compelling evidence that at least for some
GRBs a non-ignorable fraction of the energy was released in the form of
Poynting-flux, confirming the finding firstly made in the reverse-forward shock
emission modeling of the optical afterglow of GRB 990123 (Fan et al. 2002;
Zhang et al. 2003).
