We controlled the in-plane orientation of pentacene grains by graphoepitaxy using patterned amorphous-SiO2/Si substrates on which periodic grooves with slope edges had been formed. Pentacene crystals exhibited a clear tendency to align their b-axes perpendicular to the groove edges. Organic thin-film transistors were fabricated on the patterned and flat substrates and their transistor characteristics were compared. Although the patterned substrates increased the apparent mobility by only 10–20%, the number of grain boundaries with high potential barriers to carrier transport was reduced to half of that on flat substrates. This improvement is expected to enhance the response speed of pentacene organic thin-film transistors and suppress the sensitivity of their characteristics to the operating temperature.