Growth of Ru- and RuO2-composite (ROC) nanodots on atomic-layer-deposited Al2O3 film has been studied for the first time using ion-beam sputtering followed by post-deposition annealing (PDA). X-ray photoelectron spectroscopy analyses reveal that RuO2 and Ru co-exist before annealing, and around 10% RuO2 is reduced to metallic Ru after PDA at 900 degrees C for 15 s. Scanning electron microscopy measurements show that well-defined spherical ROC nanodots are not formed till the PDA temperature is raised to 900 degrees C. The mean diameter of the nanodots enlarges with increasing PDA temperature whereas the nanoclot density decreases, which is attributed to coalescence process between adjacent nanodots. It is further illustrated that the resulting nanodot size and density are weakly dependent on the annealing time, but are markedly influenced by the decomposition of RuO2. In this article, the ROC nanodots with a high density of 1.6 x 10(11) cm(-2), a mean diameter of 20 nm with a standard deviation of 3.0 nm have been achieved for the PDA at 900 degrees C for 15 s, which is promising for flash memory application.