The avalanche breakdown characteristics of a graded p+-n junction formed with aluminum ion-implantation for 4H-SiC were investigated. The breakdown voltage of the graded p+-n junction was calculated using a commercial process/device simulator and considering the ion-implanted distribution of aluminum. To compare the calculated results to the experimental results, a p+/n/n+ diode with an aluminum ion-implanted p+-layer was fabricated on a 2.8-μm-thick 1.1 × 1017-cm-3 n-type epitaxial layer. The breakdown voltage of the fabricated diode showed a higher breakdown voltage than that of the calculation. The cause of the difference in the breakdown voltages between the fabricated diode and the calculation is discussed.