PURPOSE: To obtain the subject composition, excellent in electric conductivity, electromagnetic wave shielding properties, airtightness, heat, oil anti-freeze, chemical and solvent resistance, etc., by blending and mixing a synthetic and natural rubber with specific graphitized carbon fiber of a vapor-prowth method in a specified amount.

CONSTITUTION: The objective composition obtained by blending and mixing (A) a synthetic rubber, such as acrylic, styrene butadiene rubber, chloroprene rubber, fluororubber, epichlorohydrin rubber or silicone rubber, and natural rubber with (B) 1-99% preferably heat-resistant graphitized carbon fiber having 3-500μequiv./g acidic functional groups prepared by a vapor growth method. The above-mentioned composition is excellent in also again corrosion resistance and mechanical strength and useful for hermetically sealing jointed surfaces of box or tubular units, etc., internally mounting electronic devices.