  We investigate structure of self-gravitating disks, their fragmentation and
evolution of the fragments (the clumps) using both analytic approach and
three-dimensional radiation hydrodynamics simulations starting from molecular
cores. The simulations show that non-local radiative transfer determines disk
temperature. We find the disk structure is well described by an analytical
model of quasi-steady self-gravitating disk with radial radiative transfer.
Because the radiative process is not local and radiation from the interstellar
medium cannot be ignored, the local radiative cooling would not be balanced
with the viscous heating in a massive disk around a low mass star. In our
simulations, there are cases in which the disk does not fragment even though it
satisfies the fragmentation criterion based on disk cooling time ($Q \sim 1$
and $\Omega t_{\rm cool}\sim 1$). This indicates that at least the criterion is
not sufficient condition for fragmentation. We determine the parameter range
for the host cloud core in which disk fragmentation occurs. In addition, we
show that the temperature evolution of the center of the clump is close to that
of typical first cores and the minimum initial mass of clumps to be about a few
Jupiter mass.
