  The quantum oscillations of the magnetoresistance under ambient and high
pressure have been studied for WTe$_2$ single crystals, in which extremely
large magnetoresistance was discovered recently. By analyzing the Shubnikov-de
Haas oscillations, four Fermi surfaces are identified, and two of them are
found to persist to high pressure. The sizes of these two pockets are
comparable, but show increasing difference with pressure. At 0.3 K and in 14.5
T, the magnetoresistance decreases drastically from 1.25 $\times$ $10^5$\%
under ambient pressure to 7.47 $\times$ $10^3$\% under 23.6 kbar, which is
likely caused by the relative change of Fermi surfaces. These results support
the scenario that the perfect balance between the electron and hole populations
is the origin of the extremely large magnetoresistance in WTe$_2$.
