  Recent theoretical works suggest that the pebble accretion process is
important for planet formation in protoplanetary disks, because it accelerates
the growth of planetary cores. While several observations reveal axisymmetric
sharp gaps in very young disks, which may be indicative of the existence of
planets. We investigate the possibility of planet formation via pebble
accretion in much earlier phases, the gravitationally unstable disks of class
0/I young stellar objects. We find that under the conditions of the class 0/I
disks, the pebble accretion timescales can be shorter compared to the typical
protoplanetary disks due to larger gas and dust accretion rate, but also find
that the accretion timescale is not always a decreasing function of the gas
accretion rate. By using estimated accretion timescales, we give a required
initial mass to form cores of gas giants within the lifetime of class 0/I
phases under several parameters, such as radial distances from the host star,
gas accretion rates, and dust-to-gas mass ratio. In the most optimistic case,
for example the dust-to-gas mass ratio is $f=3f_{\rm solar}$,
$\sim10^{-4}M_{\oplus}$ objects at 10 au can grow to $10M_{\oplus}$ cores
during the typical lifetime of the class 0/I phases, 0.5 Myr.
