Lithium-ion batteries have been widely used as a power source for electric vehicles and the lack of accurate estimation for the state of health (SOH) has seriously affected the practical applications of lithium-ion battery packs, so it is very imperative to find a good solution. In this paper, we propose an accurate estimation method for the SOH by using a fuzzy logic system. We first perform charging and discharging experiments for the lithium-ion batteries to obtain the residual capacitance and equivalent DC resistance under different SOHs. Then a fuzzy logic system is figured out according to the relationship between the equivalent DC resistance and the SOH. The SOH of the batteries is estimated online through inputting the calculated equivalent DC resistance into the fuzzy logic system. Experimental results show that this method is very effective to calculate the SOH of lithium-ion batteries.