Thermal properties and crystalline structure of liquid crystalline (LC) poly(ethylene terephthalate-co-2(3)-chloro-1,4-phenylene terephthalate) [copoly(ET/CPT)] were investigated using differential scanning calorimetry (DSC), thermogravimetry (TGA), limiting oxygen index (LOI) measurement, electron dispersive X-ray analysis (EM), X-ray diffractometry, and infrared spectrometry (IR). The thermal transition temperatures of copoly(ET/CPT) were changed with the composition. Copoly(ET/CPT) showed two thermal decomposition steps and the residues at 700degreesC and LOI values of copoly(ET/CPT) were almost proportional to its chlorine content. The activation energy of thermal decomposition of LC units was very low compared to that of poly(ethylene terephthalate)(PET) units. Crystal structure of copoly(ET/CPT) (20/80) was of triclinic system with the lattice constants of a = 9.98 Angstrom, b = 8.78 Angstrom, c = 12.93 Angstrom, alpha = 97.4degrees, beta = 96.1degrees, and gamma = 90.8degrees, which is very close to that of poly(chloro-p-phenylene terephthlate) (PCPT) with the lattice constants of a = 9.51 Angstrom, b = 8.61 Angstrom, c = 12.73 Angstrom, alpha = 96.8degrees, beta = 95.4degrees, and gamma = 90.8degrees. When copoly(ET/CPT)(50/50) was annealed at 220degreesC in vacuum, crystallization induced sequential reordering (CISR) was not observed but the heat of fusion was slightly increased due to the increase of the trans isomer content in PET units.