An equation of structural changes in stretching cotton elastic braid was derived by modeling a structure of cotton elastic braid consisting of latex rubber thread as core material, and the structure changes under manufacturing conditions and the effect of the changes on the physical properties were studied. The results obtained are summarized as follows; 1) The strain ratio of the core rubber thread(u) is dependent upon spiral length ratio(${eta}$). In other words, the strain ratio is dependent upon manufacturing braiding angle($ heta$a), manufacturing drafting ratio(Da) and diameter ratio of covering yarn to core rubber thread ($phi$). ${mu}$ shows large values when $ heta$a is decreased to maintain uniformity approaching D=1, and the strain ratio decreases linearly with increasing tan $ heta$a. and the width decreases significantly and the braiding angle decreases linearly, respectively depending upon the drafting ratio of the braid(DB). 3) Results from comparison of the theoretical equations(4) and (8) for width and braiding angle with the experimental values approach the theoretical equation when drafting ratio of core rubber thread(D) and $ heta$a increase. It is anticipated that this fact may be caused by the effect of ${eta}$. 4) When the structural changes of the elastic braid are considered on the basis of the conception of coefficients of width change(Kw) and braiding angle change(K$ heta$), Kw is dependent upon $ heta$a and K$ heta$ is independent upon both $ heta$a and $phi$. 5) When the Da is constant, linear relation between K$ heta$ and Kw/D is established and its slope corresponds to Da, nearly coinciding with the experimental curves in Fig. 16. 6) Estimated equations(12) for the manufacturing braiding angle shows optimum conditions within ranges of $phi$=0.40 to 0.47 and $ heta$a=20$^{circ}$ to 25$^{circ}$.