An extrusion molding silicon rubber composition including (A) 100 pbw of an organopolysiloxane, (B) 5-500 pbw of finely divided silica filler, (C) 1-50 pbw of powder boron nitride having a mean particle size of up to 20 .mu.m, and (D) a curing catalyst is extrusion molded and cured into an insulating article which has improved physical properties and dielectric strength and is thus suitable as electrical insulation.