A silicone rubber composition containing 0.5-90% by weight of a metal powder and curable into a non-conductive silicone rubber having a volume resistivity of at least 1.times.10.sup.9 .OMEGA.cm is useful for the sealing and encapsulation of electric and electronic parts because the metal powder is sulfided with a sulfur gas into a metal sulfide powder which prevents or retards the sulfur gas from reaching the electric and electronic parts.