This study proposes an analytical model to relate the evolving microstructure of neat cement paste during hydration to the electrical resistivity measurement. The model is based on the general effective media theory and considers the contributions of both the low-resistivity phase (capillary pores) and the high-resistivity phase (gels and solids) in cement-pastes. The major parameters in the proposed model are the microstructural properties such as capillary pore percolation threshold and pore structure parameter, capillary pore phase and solid-gel phase resistivities. To validate the model, an empirical relationship between the resistivity formation factors and the phase volume fractions of a series of cement pastes has been built up with the assistance of noncontact electrical resistivity measurement and mercury intrusion porosimetry technique. The good agreement between the model and experimental data shows that it is feasible to obtain better understandings of the evolving microstructure of cement paste during the hydration using electrical resistivity measurement. Furthermore, based on the log-linear increase of resistivity with time, the porosity up to an age of a few months can be predicted by the proposed model. The predicted results show a good correlation with the experimental results in previous studies. © 2009 ASCE.