In this study, the influences of coarse aggregate to the electrical resistivities of concrete were investigated. Specifically, types and sizes of the aggregates were targeted as the studied parameters. Commercially available granites and marbles with various sizes were used in the concrete mixtures, and resistivities of the specimens at the age of 28 days were investigated. Polarization induced resistivity growth during the electrical measurement were considered and experimentally nullified, such that the real bulk resistivities of concrete can then be obtained for comparison. Concrete specimens were prepared with various amounts of coarse aggregate contents and sizes; with a unique size used in a single specimen. Cement mortars were mixed with a water/cement ratio of 0.4, and the cement/sand ratio of 1. In addition to electrical measurements, microstructures of certain specimens were also investigated by Scanning Electron Microscopy (SEM) so as to estimate the porosity of the cement matrices, which is considered as the primary conductive portion of cementitious materials. The results show that the electrical resistivities of concrete composed of granites were very close to those made of marbles. The particle size of aggregates also seems to have no significant influences to concrete resistivities while it is to the strengths. As a partial study of nondestructive testing (NDT) to the performance estimations of concrete, we have drawn a few comments for future studies.