Co-carbonization properties of toluene soluble (TS) of coal tar pitch and waste polystyrene (WPS) were studied in a tube bomb to correlate the content of alkyl groups in the mesophase pitches with coefficient of thermal expansion (CTE) and anisotropic orientation. The alkyl contents, which were increased from 12.0% to 33.3% by adding WPS into TS, improved the properties of resultant needle coke in terms of optical texture and CTE value. The anisotropic indices of average length of vectors parallel to the CTE axis and average length of anisotropic unit vectors increased from 20.8 μm to 28.4 μm and 23.4 μm to 28.8 μm, respectively, and CTE value decreased from 0.8×10-6 /°C to 0.1×10-6 /°C. Due to the increasing alkyl groups, the lower viscosity of the carbonization system favored the development of flow texture and uniaxial orientation. And the sufficient gas evolution of good timing in co-carbonization forced the uniaxial arrangement of bulk mesophase molecules at the solidification stage.