A modified non-contact electrical resistivity measurement is proposed to determine porosity and pore network connectivity of cement-based materials. The porosity of mortar with different water-to-cement ratios was tested by mercury intrusion porosimetry (MIP) and the tortuosity was calculated from pore entrapment volume fraction. The relationship between pore structure and resistivity was studied using the three most commonly used theories. It was observed that the multi-phase phenomenological model can indicate the effect of porosity and pore connectivity on the resistivity of cement-based materials. The parameter m in Archie's Law as an index of pore connectivity can be expressed as a variable that varies with the tortuosity. The parameters in the GEM model are closely related to the nature of the material: the parameter M should be related to the composition of the material and the degree of hydration of the cement, and the parameter t is a variable that varies with the pore connectivity. Based on these theories, equations are derived to correlate the porosity and pore network connectivity to the formation factor of cement based materials. The developed approach thus has a potential to be used as a simple and effective tool for pore structure investigation of cement based materials.