Very thin metallic films are susceptible to dewetting upon thermal excursions, resulting in fragmentation and hence loss of structural integrity. Herein, 15 to 55 nm thick Cu films deposited on a Si substrate were isothermally annealed at 400 to 700 °C inside a scanning electron microscope operating in high-vacuum mode and the ensuing dewetting behavior was studied. The in situ observations revealed that the induction time before the void nucleation varied with film thickness as per a power-law with an exponent of 4, and the activation energy for both the void nucleation and the growth was close to the activation energy for surface diffusion. Hillock formation was observed to be a prerequisite for void nucleation in relatively thicker films. To complement the experimental observations, phase-field simulations incorporating a grain boundary grooving model were performed, which showed excellent agreement with the experimental observations. This validates the surface diffusion-controlled, grain boundary grooving-driven mechanism for void nucleation and dewetting of Cu films deposited on Si.