  The magnetoresistance (MR) for relativistic electrons, i.e., the Dirac
electrons in solids, is investigated on the basis of the Boltzmann's theory.
The new formula of MR so obtaiend includes a relativistic correction, which has
not appeared in the conventional Boltzmann's theories. Our formula is
quantitatively consistent with that obtained by the Kubo theory except for the
quantum oscillations. While a field dependence of MR is unclear in the formula
by Kubo theory, a clear field dependence is obtained in our formula, which is
useful for the analysis of experimental results. The effects of the
relativistic correction on the MR for the one-band and two-band model are
discussed. By taking into account the field dependence of carrier density in
semimetals, the linear field dependence of MR is explained by our formula based
on the Boltzmann's theory
