Electrical resistivity measurement was used to study the early setting and hardening process of pastes, with different dosages, for 2 types of superplasticizer. The inflection point (P) was found on the differential curve of electrical resistivity for each sample. A time ratio, K, is defined as the inflection time ratio of the pastes with superplasticizer to the paste without superplasticizer and a resistivity ratio, K, is defined as the ratio of resistivity of the pastes with superplasticizer to the resistivity of paste without superplasticizer at 24 h. The results show that K is linearly positive to setting time and K is linearly positive to compressive strength at 24 h. The criterion for the selection of superplasticizer is proposed using these definitions. The most suitable superplasticizer is one which gives the mixture a higher K with a longer setting time and a higher K with a rapid strength gain in the hardening period for the saturation dosage at a fixed water cement ratio. © 2007 Elsevier Ltd. All rights reserved.