A new method is presented for analyzing fiber orientation distribution in fiber reinforced composites. A fiber orientation state in fiber reinforced composites is first obtained from soft X-ray image by making contract micro-radiograph (CMR). The distribution of the fiber orientation is measured by a fiber orientation index G(theta) as proposed in this paper. Furthermore, a computer simulation for the fiber orientation distribution is carried out in order to evaluate the fiber orientation index G(theta). It has been found that the value of the fiber orientation G(theta) decreases with an increase in the scatter of the fiber orientation distribution. Not only the direction but also the degree of the fiber orientation can be shown from the curve of the fiber orientation index G(theta). The result shows that this method is easy and useful to evaluate the fiber orientation distribution in fiber reinforced composites.