The surface microstructure of c-axis (Ca,Sr)CuO2 thin films, grown by laser molecular beam epitaxy on SrTiO3(001) substrates, was studied by ultrahigh-vacuum scanning tunneling microscopy (STM). Images were obtained for codeposited Ca1-xSrxCuO2 thin films, which show a layered-type growth mode, The surfaces consist of atomically flat terraces separated by steps that are one unit cell high. A pronounced dependence of the growth mechanism on the Sr/Ca ratio of the films was observed. Atomic resolution STM images of the CuO2 sheets in the ab plane show a square lattice with an in-plane spacing of 4 angstroms; the lattice contains different concentrations of point defects, depending on the polarity of the sample-tip bias.