We have investigated the structural and electrical characteristics of two kinds of block co-oligomers, 5,5"-Bis(2-pyridyl)-2,2':5',2"-terthiophene (5A) and 2,5-Bis(2-(2'-thienyl)-pyridine-5-yl)thiophene (5B), which are composed of electron-donating thiophene and electron-withdrawing pyridine rings. At the view of building block units, the amount of the module units included in these molecules is completely equivalent to each other. X-ray diffraction patterns of 5A and 5B thin films grown on atomically flat alpha-Al2O3 (0001) substrates indicated a high degree of orientation along the c-axis. Field-effect transistors (FETs) of 5A and 5B thin films were fabricated and evaluated without exposure to air. The FETs based on 5A active layers exhibited p-type behavior with a mobility of similar to 10(-3)cm(2)/V.s and an on-off ratio of 10(4), while no switching behavior was observed in FETs based on 5B active layers. The lower highest occupied molecular orbital (HOMO) of 5B than that of 5A in the quantum-chemical calculation might explain these FET behaviors. Thus, HOMO, lowest unoccupied molecular orbital (LUMO) levels and FET actions of the co-oligomer molecules that consist of the same amount of building blocks can be controlled by the sequence of electron-donating and electron-withdrawing building blocks. (C) 2012 Elsevier B.V. All rights reserved.