Abstract: A fabricating method of an ion sensitive field effect transistor (ISFET) with a Ta.sub.2 O.sub.5 hydrogen ion sensing membrane, which comprises the steps of forming Ta.sub.2 O.sub.5 film with a thickness of about 400 to 500 A by RF reactive sputtering on a Si.sub.3 N.sub.4 /SiO.sub.2 gate dielectric layer of the pH-ISFET, and annealing the resultant film at about 375.degree. to 450.degree. C. in oxygen gas ambience for about one hour. In forming the Ta.sub.2 O.sub.5 film on the pH-ISFET, the Ta.sub.2 O.sub.5 film formed in the area except the gate region of the pH-ISFET is removed by a lift-off process utilizing a positive PR film. The Ta.sub.2 O.sub.5 gate pH-ISFET according to the present invention has higher sensitivity and more stable operation characteristics than those of the conventional pH-ISFET, while the productivity and stability thereof are greatly improved by effecting a whole wafer process.