  In this study, the light curves and spectrum of the photospheric thermal
radiation from ultrarelativistic gamma-ray burst (GRB) jets are calculated
using 2D relativistic hydrodynamic simulations of jets from a collapsar. As the
jet advances, the density around the head of the jet decreases, and its Lorentz
factor reaches as high as 200 at the photosphere and 400 inside the
photosphere. For an on-axis observer, the photosphere appears concave shaped
due to the low density and high beaming factor of the jet. The luminosity
varies because of the abrupt change in the position of the photosphere due to
the internal structure of the jet. Comparing our results with GRB090902B, the
flux level of the thermal-like component is similar to our model, although the
peak energy looks a little bit higher (but still within a factor of 2). From
the comparison, we estimate that the bulk Lorentz factor of GRB090902B is
$\Gamma \sim 2.4 \times 10^2 (r/10^{12} \rm cm$) where $r$ is the radius of the
photosphere. The spectrum for an on-axis observer is harder than that for an
off-axis observer. There is a time lag of a few seconds for high energy bands
in the light curve. This may be the reason for the delayed onset of GeV
emission seen in GRB080916C. The spectrum below the peak energy is a power law
and the index is $2.3 \sim 2.6$ which is softer than that of single temperature
plank distribution but still harder than that of typical value of observed one.
