  We perform a large set of radiation hydrodynamics simulations of primordial
star formation in a fully cosmological context. Our statistical sample of 100
First Stars show that the first generation of stars have a wide mass
distribution M_popIII = 10 ~ 1000 M_sun. We first run cosmological simulations
to generate a set of primordial star-forming gas clouds. We then follow
protostar formation in each gas cloud and the subsequent protostellar evolution
until the gas mass accretion onto the protostar is halted by stellar radiative
feedback. The accretion rates differ significantly among the primordial gas
clouds which largely determine the final stellar masses. For low accretion
rates the growth of a protostar is self-regulated by radiative feedback effects
and the final mass is limited to several tens of solar masses. At high
accretion rates the protostar's outer envelope continues to expand and the
effective surface temperature remains low; such protostars do not exert strong
radiative feedback and can grow in excess to one hundred solar masses. The
obtained wide mass range suggests that the first stars play a variety of roles
in the early universe, by triggering both core-collapse supernovae and
pair-instability supernovae as well as by leaving stellar mass black holes. We
find certain correlations between the final stellar mass and the physical
properties of the star-forming cloud. These correlations can be used to
estimate the mass of the first star from the properties of the parent cloud or
of the host halo, without following the detailed protostellar evolution.
