The electrical resistivity method was used for measuring the electrical resistivity of cement paste incorporating with fly ash. The study found that the bulk electrical resistivity ρ(t) was a function of the solution electrical resistivity ρ(t) and porosity Φ. A two-component model was proposed, in which ρ(t) was dominated by ρ(t) at the early period of hydration and dominated by Φ at a later period. The porosity formation curve was derived from the 2-point method. A logarithmic equation ρ(t) =K*ln(t/t) was proposed to express the electrical resistivity development with time after hardening, where K represents the rate of hydration, and the pastes with fly ash had a lower K. The setting times and compressive strengths were tested and the results were compared with the electrical properties. © 2004 RILEM. All rights reserved.