An infinite domain of fluid with initially-uniform temperature is considered within which a spherical heat source is suddenly activated. Prior to the occurrence of natural convection, the temperature field evolves exclusively via diffusion. In this study, the transient conduction process in nanofluids is examined by the energy integral equation together with the dual-phase-lag model. The results indicate that the relative thermal penetration depth is strongly affected by the nanoparticle size and volume fraction. At a volume fraction of and a time of , for instance, and nanofluids exhibit relative thermal penetration depths of and , respectively, for a nanoparticle diameter of . Under the same conditions, the corresponding magnitudes for a nanoparticle diameter of are nearly and for aluminum oxide and copper oxide nanofluids, respectively. Increasing the volume fraction from to in alumina nanofluid with a nanoparticle diameter of results in an approximate increase in the relative thermal penetration. A stabilization of temperature evolution is sometimes observed when the thermal front no longer propagates further in nanofluid.