Solid state structure has been studied for a series of recently synthesized liquid crystalline polycarbonates1based on methyl-substituted stilbene mesogen and methylene-containing flexible spacer. These polymers are referred to as HMS-n, wherenis the number of methylenes in the flexible spacer group. The stability of the liquid crystalline phase and its relationship to the three dimensional crystalline phase were studied. X-ray diffraction patterns of raw fibers drawn from the mesophase revealed that the stability of the mesophase decreases as the methylene spacer length gets close to that of mesogen. From X-ray diffraction patterns of annealed fibers and molecular modeling studies, the unit cell parameters of HMS-5 to 8 were determined. HMS-5 and 6 have an orthorhombic structure while HMS-7 and 8 have a monoclinic structure. More important, an odd-even oscillation is observed in thed-spacings of the (020) and (110) reflections as a function ofn, which relates to differences in interchain packing in the odd and even members of the series. All HMS-5 to 8 have a stable intermeshed crystal structure in which the mesogen and the flexible spacer group on adjacent chains are aligned. In this structure, the disposition of the carbonate group differs fromn-even ton-odd, and is responsible for the odd-even effect seen in the two dominant interchaind-spacings. We suggest the higher degree of overlap of the carbonate linkage of HMS-7 and 8 might be the reason for their less stable mesophase compared with HMS-5 and 6.