The characteristics and feasibility of the dielectric can be evaluated by the deposition of the dielectric in between two electrodes to form a capacitor. In metal-insulator-silicon (MIS) structure, 100 Å Ta2O5 films are deposited on 1000 Å doped poly silicon as a bottom electrode layer and 500 Å TiN metal on top of the dielectric film consecutively . The measured capacitances of larger areas roll off at higher measuring frequency . With the setting of an equivalent circuit, one successfully explains why the roll-off effect shows up. The deduced formulas give the common, reasonable, and convincing C1=1.1255×10-5 nF/mm2, which determines the equivalent dielectric (oxide) thickness 30.5Å.