SiC MIS structure with ultra-thin Al2O3 as gate dielectric deposited by Atomic Layer Deposition(ALD) on epitaxial layer of 4H-SiC(0001)80N-/N+ substrate is fabricated. The experimental results indicate that the prepared ultra Al2O3 gate dielectric exhibits good physical and electrical characteristics, including a high breakdown electrical field of 25MV/cm, excellent interface properties(2×1013cm-2•eV-1) and low gate-leakage current (IG=1×10-3A/㎝-2@EOX=8MV/cm). Analysis of current conduction mechanism in deposited Al2O3 gate dielectric has also been systematically performed. The confirmed conduction mechanisms consisted of FP emission, FN tunneling, DT and Schottky emission. And the dominance of these current conduction mechanisms depended on applied electrical field, When the gate leakage current mechanism is dominated by FN tunneling, the barrier height of SiC/Al2O3 is 1.4eV, which can meet the requirement of SiC MISFET devices.