  The inner region of a luminous accretion disk is radiation pressure
dominated. We estimate the surface temperature of a radiation pressure
dominated accretion disk, \Theta=(c_s/r\Omega_K)^2<<(H/r)^2, which is
significantly lower than that of a gas pressure dominated disk, \Theta (H/r)^2.
This means that the outflow can be launched magnetically from the photosphere
of the radiation pressure dominate disk only if the effective potential barrier
along the magnetic field line is extremely shallow or no potential barrier is
present. For the latter case, the slow sonic point in the outflow may probably
be in the disk, which leads to a slow circular dense flow above the disk. This
implies that hot gas (probably in the corona) is necessary for launching a jet
from the radiation pressure dominated disk, which provides a natural
explanation on the observational evidence that the relativistic jets are
related to hot plasma in some X-ray binaries and active galactic nuclei. We
investigate the outflows accelerated from the hot corona above the disk by the
magnetic field and radiation force of the accretion disk. We find that, with
the help of the radiation force, the mass loss rate in the outflow is high,
which leads to a slow outflow. This may be the reason why the jets in
radio-loud narrow-line Seyfert galaxies are in general mild relativistic
compared with those in blazars.
