Organic field effect transistors (OFETs) have attracted much attention as key devices to realize plastic electronics. Low mobility of OFETs is one of the major problems for practical use. To improve the mobility, fabrication technique of high quality organic semiconductor thin films is important. Surfaces of gate insulators are amorphous in many cases, so that a novel technique to control in-plane orientation on such amorphous substrates is urgently required. In this report, application of "graphoepitaxy" to control the in-plane orientation is proposed. The authors discovered that organic semiconductor α-sexithiophene grows with in-plane preferred orientation on artificial periodic grooves fabricated on thermally-oxidized silicon substrates. Using this technique, the test devices of OFETs with oriented 6T thin films were prepared. The mobility has not yet been improved owing to morphological problems of the thin films. A guide to the next step of research is presented based on current imaging by conductive atomic force microscopy.