The first stars fundamentally transformed the early Universe by emitting the first light and by producing the first heavy elements. These effects were predestinated by the mass distribution of the first stars, which is thought to be fixed by a complex interplay of gas accretion and protostellar radiation. In this talk, I present our recent work of radiation-hydrodynamics simulations that follow the growth of a primordial protostar through the early stages of a star with thermo-nuclear burning. Our simulations of this "massive star formation in the early universe" show the strong stellar UV feedback finally shuts off the mass accretion and limit the final stellar mass. We predict a mass distribution of about 100 first stars, which ranges from 10s to 100s Msun depending on physical properties of star-forming clouds.
