It is uncommon for a solid substance to expand upon cooling and contract upon heating. In this work, we synthesized a carbocycle monomer that contains 1,2:5,6-dibenzocyclooctadiene (DBCOD) unit and the corresponding polymers with different amounts of DBCOD in the polymer main chain. The films produced from this DBCOD-based polymer showed reversible thermal contraction behavior with the negative thermal expansion (NTE) coefficient of -625 ppm/K (100-200 degrees C). Differential scanning calorimetry (DSC), variable-temperature nuclear magnetic resonance (VT-NMR), and density-functional theory (DFT) calculations indicated that the population of the chair conformer of DBCODs increased with the rise of temperature due to the conversion from twist-boat to chair conformer. This led to giant thermal contraction. Furthermore, the value of the NTE coefficient could be adjusted by the amount of DBCODs and their surrounding physical environment. This is the first demonstration of a linear polymer that exhibits reversible thermal contraction. The new monomer can be covalently incorporated into a variety of polymeric materials to construct negative thermal expansion or zero expansion materials for the specific applications.