Thin films of (Ca,Sr)CuO2, the parent material of high Tc cuprate superconductors, have been formed by the laser ablation method under molecular beam epitaxial condition, and the growth mechanism has been investigated with reflection high‐energy electron diffraction (RHEED). Analyses of RHEED patterns and intensity oscillations show that this material grows with two‐dimensional layer growth. When all the metal elements are supplied simultaneously in NO2 atmosphere, the layer growth occurs with the unit‐cell layer of (Ca,Sr)CuO2. Furthermore, it has become evident that the growth unit can be separated into Ca (Sr) atomic layer and CuOx atomic layer by monitoring the RHEED intensity oscillation. The successive supply of each metal element leads to one atomic layer growth of this material.