Silicones are widely used for electrical insulation owing to their high dielectric strength and thermal stability. However, recent studies revealed insufficient stability of silicone for high-temperature applications. To study the effect of Al2O3 fiber on silicone stability, we measured the dielectric strength of unfilled silicone and Al2O3/silicone composites as a function of aging time at 250°C in air and analyzed data by Weibull probability distribution to determine characteristic dielectric strength (E0) and shape parameter (β). Prior to aging, unfilled silicone and composites had similar behavior, with E0 at about 20 kV/mm and β > 15. During aging, unfilled silicone developed both micro- and macrocracks, with β dropped below five in 240 h and E0 decreased significantly. Composites developed microcracks, with β dropped below 5 in longer time and E0 remained almost constant. Addition of Al2O3 slowed down crack growth in silicone matrix, resulting in longer lasting high-temperature dielectric materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41170.