We developed an ion sensor with a novel structure that does not require a reference electrode. The structure consists of a double electrode comprising an outer electrode with a hole and an inner electrode, a polymeric ion-selective membrane, and the sensor body. The ion sensitivity of a potassium ion sensor, where the hole of the outer electrode was the same size as the inner electrode, was 55 mV/decade. The sensor exhibited increased sensitivity according to the increment of the hole size. The sensitivity was also dependent on the thickness of the outer membrane covering the outer electrode. These phenomena were examined using a membrane potential gradient simulation. Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America.