The low-temperature electrical transport properties of nanocrystalline boron-doped diamond (b-NCD) thin films have been found to be strongly affected by the system's granularity. The important differences between the high and low-temperature behaviour are caused by the inhomogeneous nucleation of superconductivity in the samples. In this paper we will discuss the experimental data obtained on several b-NCD thin films, which were studied by either varying their thickness or boron concentration. It will be shown that the low-temperature properties are influenced by the b-NCD grain boundaries as well as by the appearance of an intrinsic granularity inside these granules. Moreover, superconducting effects have been found to be present even in insulating b-NCD films and are responsible for the negative magnetoresistance regime observed at low temperatures. On the other hand, the low-temperature electrical transport properties of b-NCD films show important similarities with those observed for granular superconductors. © 2010 IOP Publishing Ltd.