Models for the crystal structure of cis-1,4-poly(2,3-dimethyl-1,3-butadiene) are elucidated by the joint use of molecular mechanics and WAXS. The chain conformation can be predicted by using different sets of potential functions. A model of crystal structure calculated by molecular mechanics has been refined in order to find the best accordance with the experimental X-ray powder spectra. Another model is proposed to explain a few minor discrepancies between the calculated and the experimental WAXS powder spectra.