Nickel phthalocyanine(NiPc) sensing films for O@sub 3@ gas have been fabricated by r.f.- plasma-activated evaporation and common vacuum deposition techniques. Sensor structure as a detecting element has a inter digital type Au or Pt electrode deposited on a glass substrate. Sensing NiPc films formed on the electrode are about 400 and 800 Å in thickness. Original value of electric resistance of the NiPc films between the electrodes is about 10@super -10@-10@super -11@ @ohm@ , but exposing the sensing film to O@sub 3@ gas it immediately changes down to 5-6 order of magnitude under the applied voltage d.c.1.5 V. Response time of the current corresponding to 63% of the saturated value (time constant 1 @tau@) is about 3.5 min. Due to more thinner NiPc films response time can be shorten and saturate more quickly. In this report we showed the data of only with 150 ppm but this kind of sensors can be detectable it more lower than 5 ppm. From the results of response characteristics, it suggests that absorbed O@sub 3@ gases on the NiPc film diffuse inside the film, so that the response time of cause, depends on the film thickness and structure. Comparing the response times with 400 and 800 Å, thinner one is shorter several times than the other, and also the films prepared by plasma-activated evaporation respond quickly than that the films without plasma-activation. To clarify the reason of different responses, morphologic images of the NiPc films were observed by Atomic Force Microscope. Surface of the films formed by common vacuum deposition is likely needle crystals but the other films show very flatness.