A method for manufacturing heat-resistant reinforced composite materials having a fibrous structure which comprises mixing 99.8 to 90 percent by weight of a first component selected from the group consisting of the nitrides or carbides of aluminum, silicon and boron and mixtures thereof and 0.2 to 10 percent by weight of a second component selected from the group consisting of the rare earth oxides and mixtures thereof, molding the mixture into a desired shape, and sintering the molded mass to create a fibrous structure therein, said first component being 0.2 to 3.0 microns in average particle size, and said second component being 0.1 to 2.0 microns in average particle size.