  The longitudinal resistivity (rho_{xx}) and transverse resistivity (rho_{xy})
of MgB2 thin films in the mixed state were studied in detail. We found that the
temperature dependencies of rho_{xx} and \rho_{xy} at a fixed magnetic field
(H) satisfy the scaling law of $\rho_{xy}=A\rho_{xx}^\beta$, where the exponent
beta varies around 2.0 for different fields. In the low field region (below
1T), beta maintains a constant value of 2.0 due to the weak pinning strength of
the vortices, mainly from the superfluid of the pi band. When H>1T, beta drops
abruptly to its lowest value at about 2T because of the proliferation of
quasiparticles from the pi-band and, hence, the motion of the vortices from the
superfluid of the sigma-band dominates the dissipation. As the field is
increased further, the vortex pinning strength is weakened and beta increases
monotonically towards 2.0 at a high field. All the results presented here are
in good agreement with the expectation of the vortex physics of a multi-band
superconductor.
