In recent years, it has been increasing in demand for high performance CMCs for high temperature application. CMCs are most promising materials for high temperature structural materials of gas turbines for aerospace and power generation. Mechanical performances of CMCs are highly dependent on properties of the reinforcement. The oxygen free SiC fiber (Hi-Nicalon) has been commercially produced by an electron beam curing process. And then the SiC fiber (Hi-Nicalon Type S) having stoichiometric SiC composition and high crystallinity was developed. Hi-Nicalon fiber has higher elastic modulus and thermal stability than Nicalon fiber. The Type S fiber has the highest elastic modulus and thermal stability and excellent creep resistance in three types of Nicalon family fibers. Recently, the Type S fibers as industrial products were developed and put on the market. The Type S (industrial version) fibers had a high tensile strength of 2.8 GPa, a high elastic modulus of 390 GPa. The Type S fiber retained a tensile strength of 2.2 GPa and an elastic modulus of 390 GPa after exposure at 1873 K. Moreover, the Type S fiber had outstanding creep resistance; the Type S fiber showed higher stress relaxation ratio than many other ceramic fibers after thermal exposure over 1673 K. The Type S fibers could be the best candidate for the reinforcement of CMCs. The fibers can be supplied about 30 kg per a month at present. Now, the Type S fiber/BN/SiC composites are being developed as the components of gas turbine for aerospace and land based power generation such as shrouds and combustors.