Abstarct: According to applications of conducting polymers as the main component organic light-emitting diodes (OLED), the study of factors affecting the properties of these types of polymers has significant importance. In this study, structural, electronic and thermochemistry properties of oligo (3,4-ethylenedioxythiophene) were investigated theoretically.The DFT-B3LYP/6-311++G** and B3LYP/6-311G** level of theories were used To perform quantum calculations . The resuls of the structural calculations of the oligomers EDOT was shown that average of dihedral angles for three states neutral, cationic and anionic was to be nearly 〖180〗^° and have planer structure. By increasing chain length of oligomer, the length of conjugated single and double bonds to be more closely which indicates an increase in the electron conjugation property. Result obtained is better for doped species. Energy gap between HOMO and LUMO decreases with growth in the oligomer chain which caused an increased conductivity in larger species. This values for of both doped states (n- and p-type) has more reduction. The results show that the ease of hole and electron injection in these oligomers according to improvement properties of the doped species (n- and p-type). Besides, by increasing the oligomer chain length, electric charge distributes uniformly between the EDOT ring during an oligomer chain. Simulated UV/Vis spectrum for oligomers, indicates that the observed red shift due to both increase of chain length and doped process is accords with improvement of conductivity in poly (EDOT). The study of the temperature effect on the thermochemistry properties of these molecules was shown that the larger oligomers have more stability . Prediction of conductivity of poly (EDOT) in gas-phase at the range of 50-400 K show, that the conductivity increases with increasing temperature.