Quality of fly ash used as admixture of concrete can be evaluated by electrochemical technique. The charge transfer resistance R ct and the HF capacitance C HF represent the characteristic of pozzolanic activity and water requirement ratio of fly ash. Testing result shows that there is linear correlation between the charge transfer resistance R ct and the compressive strength for 28 days, while the HF capacitance C HF and water requirement ratio of fly ash have the relation that can be represented by fourth power polynomial.