A spark plug which includes a center electrode metal shell and an insulator disposed therebetween utilizes for the insulator or ceramic with improved high temperature electrical properties which includes alumina in an amount between about 90 and about 99% by weight, a zirconium containing compound in an amount between about 0 and about 1% by weight, and an oxide mixture in an amount between about 1 and about 10% by weight. The oxide mixture includes a glass former, a network modifier and alumina in an amount between about 16% and about 40% by weight after firing, wherein the molar ratio of the glass former to the network modifier ranges between about 0.8:1 and 1.2:1. The ceramic insulator is particularly adapted for use as an insulator in a spark plug to provide improved dielectric strength and shunt resistance of greater than one 1000 megaohms at 1000 degrees Fahrenheit, so as to reduce the shunting of the spark plug and thereby improve the quality of the spark generated by the spark plug.