This work has been carried out to find the fundamental reasons of raw silk size nonuniformity by studying with the physical nature and its bave formation through the worm spinnerate with anatomical device. Mean time, the uniformity improving method is also attached to this paper after throughfall theoretical study. 1. According to the anatomical study of the silkworm spinnerate, the physical form decides the fiber cross section and also spinns more finner size as the spinning is continued collaborating with the muscle pressure and decrease of liquid silk in the silk gland. 2. The bave size deviation of the Korean cocoons have shown a little difference among the varieties, but, they are conclued as 0.58 denier in average which caused some size deviation of the raw silk inevitably. 3. The raw silk nonuniformity increases when the cocoon have size is not suitable with the proposed raw silk size. 4. The nonuniformity is also increased very much during the filature process in case of poor technical work, but even if we do our best, the best result is found as 30 percent improvement which is still worser than the natural cocoon size deviation. 5. It is required to be careful for the selection of silkworm variety in order to improve the nonuniformity of raw silk so that the bave size be suitable for the proposed silk. 6. The definite size silk reeling method could made better uniform silk, 20 percent at least, than the definite cocoon number silk reeling method. 7. A newly developed denier(21d) gauge was applied for the multiends type silk reeling machine which is widely used in Korea, and the result of the method showed 1.30 denier size deviation, but, in case of no use of it was 1.57 denier size deviation, that is, it could improve two grades more than the normal reeling method.