The organopolysiloxane composition, e.g. silicone fluid, of the invention is imparted with high heat resistance by admixing with a specific heat-resistance improver which is prepared by a heat treatment of a mixture composed of (a) 100 parts by weight of an organopolysiloxane having a specified viscosity, (b) a carboxylate of cerium such as 2-ethylhexoate in an amount of 0.05 to 5 parts by weight as cerium and (c) an alkoxide compound of titanium or zirconium such as tetrabutyl titanate in an amount to give 0.1 to 5 times by weight of the titanium or zirconium element based on the cerium content in the component (b) at a temperature of, for example, 150.degree. to 310.degree. C. for 30 minutes to 10 hours.