Thermal transport across interfaces can play a critical role in nanosystems for thermal management and thermal energy conversion. Here, we show the dependence of the thermal boundary conductance (G) of the interface between a 70-nm Al transducer and a Si substrate on the size of a laser pump diameter (D) in the time-domain thermoreflectance (TDTR) experiments at room temperature. For D ≥ 30 μm, G approaches to a constant where diffusion dominates the heat transfer processes. When D decreases from 30 μm to 3.65 μm, G decreases from 240 to 170 MW/m2K due to the increasing nonlocal effects from nondiffusive heat transport. This finding is vital to our understanding of the thermal boundary conductance: it depends not only on inherent interfacial conditions but also on external heating conditions, which makes the accurate measurements and theoretical predictions of thermal transport across interfaces in micro/nanosystems more challenging.