Abstract: Because the forming of yarn is characterized by the changes of the space arrangement orderliness of the short fibers or the filaments, so there exists the entropy change phenomenon, and in most cases of texturing there is an increasing of the system entropy. The entropy is a state function, thus the entropy changing of the textured yarn has a close relationship with its bulk. Further theoretical derivation has showed that the texturing entropy changing is in proportion to the log of bulk index, as well as to the bulky; the disordering degree shares the same value with bulk index, so it also goes directly with the bulky, all experimental values are in good agreement with this results. Because textured yarn has a pair of equal valued disordering degree and bulk index, the former can be measured by testing the bulk index of it, further, the texturing entropy changing can be studied in quantization.