The thermally conductive composite sheet of claim 1, wherein in the liquid addition curing silicone rubber composition the organopolysiloxane, (a), is of the formula: Ra SiO.sub.(4-a)/2 wherein R is a halogen-substituted or unsubstituted monovalent hydrocarbon group, and the organohydrogen polysiloxane, (b), has 0.1 to 1.5 hydrogen atoms bound to silicon atoms per alkenyl group in the organopolysi