The present invention provides a thermally conductive grease composition which scarcely increases in hardness during high-temperature heating and has a minimized reduction in growth, containing: (A) an organopolysiloxane including at least two alkenyl groups per molecule, the 25° C. kinetic viscosity being 5,000 to 100,000 mm2/s; (B) a hydrolysable methyl polysiloxane, trifunctional at one terminus, represented by general formula (1) (R1 is a C1-6 alkyl group, and a is an integer 5 to 100); (C) a thermally conductive filler having a thermal conductivity of 10 W/m·° C. or greater; (D) an organohydrogenpolysiloxane containing two to five hydrogen atoms directly bonded to a silicon atom (Si—H group) per molecule; (E) an adhesion promoter having a triazine ring and at least one alkenyl group per molecule; and (F) a catalyst selected from the group consisting of platinum and platinum compounds.