(Ba,Sr)TiO3 thin films were prepared on SrTiO3 (001) substrates by a molecular beam epitaxy technique. (Ba,Sr)O and TiO2 monolayers were alternately grown in twenty cycles to produce films of 8 nm thick. The growth mechanism of the films was elucidated from reflection high energy electron diffraction and atomic force microscopy. An island growth mechanism was found to be predominant, and activation energies for the surface migration of Sr/Ba and Ti were determined to be 0.31 and 0.33 eV, respectively. According to coaxial impact collision ion scattering spectroscopy measurements, the atomic configuration of the film surface improved when the ratio Sr/Ba increased from Ba0.514Sr0.486TiO3.56 to Ba0.304Sr0.696TiO3.50.