[en] External electric field-induced structural changes in the initial growth processes of pentacene on a sylanol-saturated amorphous SiO_2 surface have been investigated using molecular dynamics simulations. A series of electric fields with different directions and strengths are applied to the initial lateral-orientated pentacene cluster. After that different molecular orientations are observed, which obviously present the regulatory effects of an electric field on the molecular orientation. When the electric field perpendicular to the molecular plane is applied to the pentacene cluster, the pentacene molecules keep the lying-down conformation regardless of changes in the electric field strength. Contrary to that, under the electric field parallel to the pentacene long axes or short axes with proper strength, the pentacene cluster undergoes a conformational transition from a lying-down conformation to a standing-up one. The results of this work may enrich the information on the electric field-induced orientation controllable fabrication approaches of organic semiconductor thin films. (paper)