A new mesophase semiconductor, 2,2'-dioctyl-5,5'-dithienylnaphthalene (8TNAT8) was studied on the mesophase structure by the combination of dilatometry and XRD measurements. It was found that the mesophase has 3-D lattice of triclinic with a layered structure and the molecules align in a way of a herringbone arrangement. This layered structure has a similarity to that of the crystal phase, of which XRD patterns was obtained for a thin film transistor with a top-contact/botto-gate geometry showing a fast transistor mobility (0.14cm(2)V(-1) s(-1)) at room temperature (a crystalline phase). Though the polydomain structures is formed in thin films of the transistor, the molecules are likely to totally align in a way that the molecular long axis is orthogonal to the substrate surface with a small tilt angle and this alignment of molecules is just fitting as a connector between the source and drain electrodes. This implies that mesomorphic properties could be a good character for the uniform alignment of molecules even for the crystalline phase as well as the relatively larger domain formation.