  The extremely large transverse magnetoreistance (the magnetoresistant ratio
$\sim 1.3\times10^5\%$ in 2 K and 9 T field, and $4.3\times 10^6\%$ in 0.4 K
and 32 T field, without saturation), and the metal-semiconductor crossover
induced by magnetic field, are reported in NbSb$_2$ single crystal with
electric current parallel to the $b$-axis. The metal-semiconductor crossover is
preserved when the current is along the $ac$-plane but the magnetoresistant
ratio is significantly suppressed. The sign reversal of the Hall resistivity in
the field close to the crossover point, and the electronic structure
calculation reveals the coexistence of a small number of holes with very high
mobility and a large number of electrons with low mobility. These effects are
attributed to the change of the Fermi surface induced by the magnetic field.
