Abstract: Inorganic fibers having a high strength and a high modulus of elasticity at a high temperature are provided. The inorganic fibers are amorphous inorganic silicon nitirde-based fibers composed of silicon and nitrogen, optionally with oxygen, carbon and hydrogen, and having atomic ratio between above respective elements of N/Si=0.3 to 3, O/Si=up to 15, C/Si=up to 7 and H/Si=up to 15, wherein ratios of X-ray scattering intensity thereof to that of air at 1.degree. and 0.5.degree. are from 1 to 20 respectively and the silicon nitride-based fiber is still amorphous after heating in an inactive atmosphere at 1200.degree. C. for 1 hour. Composites of a metal, a ceramics, and a high molecular weight organic compound reinforced with the above fibers are also provided.

Abstract: A high strength, high modulus pitch-based carbon fiber has a crystalline structure in which the presence of the (112) cross-lattice line and the resolution of the diffraction band into the (100) and (101) diffraction lines, which indicate the three-dimensional order of the crystallite of the fiber, are not recognized, and in which the orientation angle (.phi.) of X-ray structural parameter is not greater than 12.degree. and the stack height (Lc) ranges between 80 and 180 .ANG.. The carbon fiber also has a single-fiber diameter of 5 to 12 .mu.m, tensile strength not lower than 3.0 GPa, tensile elastic modulus not smaller than 500 GPa and elongation not smaller than 0.5%.

Abstract: Silicon nitride based ceramic fibers containing Si, N, O and at least one metal M in amounts providing the atomic ratios N/Si of 0.3 to 3, O/Si of 0.0001 to 15 and M/Si of 0.001 to 5 and exhibiting an X-ray small angle scattering such that the ratio of the intensity of the small angle scattering of the fibers to that of air is 1 to 20. The ceramic fibers are produced by spinning of a polymetallosilazane into fibers and calcining the spun fibers and are useful as reinforcing fibers for composite artices having a metal or resin matrix.