The elastic modulus (El) of crystalline regions in the direction parallel to the chain axis in a new semiaromatic polyamide 9-T (PA9-T) containing a long aliphatic chain unit was measured by X-ray diffraction and the derived El value was discussed in terms of its molecular conformation and mechanical properties. For low and high molecular weight PA9-Ts, the El value of 40 GPa was obtained. The low El value for PA9-T is due to the presence of aromatic groups with long crankshaft arms, which result in a greater moment of force during deformation compared with those polymers with planner zigzag molecular conformation like polyethylene and nylon 6 a-form. Another factor for the low El is due to the contraction of its molecular chains that is likely originated from the existence of a torsional conformation. The presence of contraction causes some internal rotation in the polymer chain during extensional deformation, which results in low El. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers