Glycated hemoglobin (HbA 1c ) is one of the most important diagnostic assays for the long-term mark of glycaemic control in diabetes. This study presents an affinity biosensor for HbA 1c detection which is label-free based on the impedance measurement, and it features low cost, low sample volume, and requires no additional reagent in experiments. The ring-shaped interdigital electrodes (RSIDEs) are designed to promote the distribution uniformity and immobilization efficiency of HbA 1c , and are further employed to characterize the impedance change and identify various concentrations of HbA 1c . The self-assembled monolayer (SAM) of thiophene-3-boronic acid (T3BA) is provided to modify the gold electrode surface. Afterwards, the esterification reaction between HbA 1c and T3BA produces a relative change of electrical property on the electrode surface. The RSIDEs with SAM of T3BA exhibit a wide range from 100 to 10ng/µL producing an approximate logarithmic decrease of impedance, a low detection limit of 1ng/µL, a good selectivity and short-term stability for HbA 1c determination. The remarkable advantages (miniaturization and low-cost) fill the bill of point-care diagnostics for portable sensor development.