We have observed the growth mode transition from two-dimensional (2D) layer-by-layer to step-flow in the earliest stage growth of heteroepitaxial SrRuO3 thin films on TiO2-terminated (001) SrTiO3 substrates by in situ high pressure reflective high energy electron diffraction (RHEED) and atomic-force microscopy. There is no RHEED intensity recovery after each laser pulse in the first oscillation when the growth mode is 2D layer-by-layer. On the other hand, it is getting more pronounced in the second oscillation, and finally reaches a dynamic steady state in which the growth mode is completely changed into the step-flow mode. The origin of the growth mode transition can be attributed to a change in the mobility of adatoms and switching the surface termination layer from the substrate to the film. SrRuO3 thin films with an atomically smooth surface grown by atomic layer control can be used in oxide multilayered heterostructure devices.