The crystal structure and chain conformation of poly[bis(3,4-dimethylphenoxy)phosphazene] - PB(dMe)PP - have been studied by X-ray diffraction techniques. The unit cell of this polymer shows an orthorhombic form with the crystallographic parameters a = 2.05, b = 1.49, c (chain axis) = 0.998 nm. Its space group is determined as Pbcn D2h14 where the molecular chains are located at the centre and each corner of the unit cell, which contains eight monomeric units; these molecular chains possibly have a -(trans3cis)2- conformation. The R factor calculated from the final crystal structure was estimated as 16.9%. As with other types of polyorganophosphazenes, the thermotropic transformation in PB(dMe)PP takes place from the three-dimensionally ordered state to its two-dimensional psuedohexagonal form (a(h) = 1.49 nm), accompanied by a latent heat change at 96-degrees-C.