Magnetoresistance (MR) of NbSe3 has been measured between 2 K and 150 K und
er high pressure including critical pressures (P-c2 and P-c1) where the low
er and upper CDW phase are totally suppressed by pressure. respectively. At
ambient pressure. a large MR (LMR) is observed just below T-c2, but no MR
is detected above T-c2. Under high pressure, in addition to the LMR. we obs
erve newly a MR above T-c2. Here we call it the pressure-induced MR (PIMR).
The LMR always appears as long as the lower CDW phase exists. but it rapid
ly disappears above P-c2. Similarly. the PIMR disappears above P-c1. Furthe
rmore. no anomalies associated with the field-induced CDW are observed with
in our experimental limits. From these findings, we claim that the LMR and
PIMR are due to normal carriers in small pockets created by a pressure-depe
ndent imperfect nesting of the Fermi surface in the CDW transitions.