We report on the formation of nanocrystalline Al thin films (180 nm thick) via magnetron sputtering technique using a step-wise deposition concept where columnar growth is inhibited, giving place to the development of a nanocrystalline mosaic grain arrangement with characteristic diameters of ≈ 30 nm and small size dispersion. The thermal evolution of the grain size distributions is investigated by transmission electron microscopy (TEM) in samples annealed in high vacuum for 3600 s. For the temperature range 300 ≤ T ≤ 462 °C the system presents a 3-D regular growth behavior up to sizes ≈ 70 nm. For T = 475 °C a rather sharp transition from normal to abnormal grain growth occurs. The grains extend to the film thickness and present mean lateral dimensions of ≈ 1000 nm. The observed phenomenon is discussed in terms of a synergetic grain boundary mobility effect caused by the characteristics of the initial nanogranular grain boundary morphology.