Nano-fibers composite ceramics were mainly composed of fiber eutectics with random orientation, in which nanometer sized second fibers are dispersed within the ceramic matrix. First, Mori-Tanaka method was used to predict the stiffness of the fiber eutectic structure. The fiber eutectic structure is transverse isotropy and has five independent elastic constants. Then considering random orientation of the fiber eutectic structure, the Young’s modulus and Poisson’s ratio of composite ceramics is determined by mean strain. Composite ceramics is isotropy. When the volume fraction of nano-fibers increase, the Young’s modulus of composite ceramics decrease and are little smaller than the volume average value, the Poisson’s ratio of composite ceramics decrease and are little bigger than the volume average value.