An organopolysiloxane composition suited for magnesium alloy bonding comprises (A) a specific organopolysiloxane, (B) a non-reactive silicone oil, (C) zinc oxide surface treated with a silane or siloxane treating agent, (D) heavy calcium carbonate surface treated with a paraffin treating agent, (E) an organosilicon compound having at least three hydrolyzable radicals or a partial hydrolyzate thereof, (F) a silane coupling agent having at least one epoxy radical, and (G) a titanium chelate catalyst. The composition has improved chemical resistance.