  We report on a systematic study of Hall effect using high quality single
crystals of type-II Weyl semimetal WTe2 with the applied magnetic field B//c.
The residual resistivity ratio of 1330 and the large magnetoresistance of
1.5\times10^6 % in 9 T at 2 K, being in the highest class in the literature,
attest to their high quality. Based on a simple two-band model, the densities
(n_e and n_h) and mobilities (\mu_e and \mu_h) for electron and hole carriers
have been uniquely determined combining both Hall- and electrical-resistivity
data. The difference between ne and nh is ~1% at 2 K, indicating that the
system is in an almost compensated condition. The negative Hall resistivity
growing rapidly below ~20 K is due to a rapidly increasing \mu_h/\mu_e
approaching one. Below 3 K in a low field region, we found the Hall resistivity
becomes positive, reflecting that \mu_h/\mu_e finally exceeds one in this
region. These anomalous behaviors of the carrier densities and mobilities might
be associated with the existence of a Lifshitz transition and/or the spin
texture on the Fermi surface.
