The improvement of the sensing performance for the mixed-potential-type acetone sensor using NASICON and Cr-based spinel-type oxide (ACr₂O₄, A = Zn, Co, Ni) sensing electrode was examined by increasing the length of three-phase boundary (TPB). Among the spinel-type oxides tested, NiCr₂O₄was found to be the best suited for the sensing electrode. The NASICON powder was mixed with NiCr₂O₄in different mass fraction (10 wt.%, 20 wt%, 30 wt.% and 40 wt.%) to improve the length of TPB. By mixing NASICON powder into the NiCr₂O₄electrode, the contact between NASICON and the sensing material existed not only at the interface between the NASICON layer and the oxide thick film, but also at the inside of the sensing electrode. As a result, a three-dimensional TPB was constructed, and showed an enhancing effect on the sensitivity of the sensor. The sensitivity of the device using NiCr₂O₄electrode mixed with 30 wt.% NASICON to 5 ppm-100 ppm C₃H₆O vapor was - 58 mV/decade at 375℃, which was higher than other devices (10 wt%, 20 wt.% and 40 wt.%). It was also observed that the sensor showed a speedy response and recovery kinetics to acetone vapor.