This invention is directed to a laser-subdivided aluminum nitride substrate wherein a resolidified layer formed at the laser score lines is constituted by one or more types of material selected from the group consisting of oxides, nitrides and oxynitrides of aluminum and of additives of said aluminum nitride substrate, and to a method of producing an aluminum nitride substrate wherein, subsequent to laser scoring the substrate, a heat treatment step at about 1000.degree. C. to 1800.degree. C. is carried out prior to the step of metallizing. This simple method provides an aluminum nitride substrate having a prescribed withstand-voltage characteristic.