  We report the observation of an unusual $B$ dependence of transverse
magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide
superconductors. Below the spin density wave transition, MR is large, positive
and increases with decreasing temperature. At low temperatures, MR increases
linearly with $B$ up to 14 T. For $T$$\geq$40 K, MR vs $B$ curve develops a
weak curvature in the low-field region which indicates a crossover from $B$
linear to $B^2$ dependence as $B$$\rightarrow$0. The $B$ linear MR originates
from the Dirac cone states and has been explained by the quantum mechanical
model proposed by Abrikosov.
