A method for manufacturing an organic thin film transistor includes steps of: forming a graphene layer on a surface of a metal substrate; covering a surface of the graphene layer with an organic solution and heating the graphene layer to form organic semiconductor nano lines on the surface of the graphene layer; and transferring the organic semiconductor nano lines to a target substrate. The graphene layer is formed on the surface of the metal substrate in mass production. The organic semiconductor nano lines (monocrystalline semiconductor) are grown in mass production by the graphene layer. The semiconductor layer having organic thin film transistors is formed after transferring the organic semiconductor nano lines on the target substrate. A large amount of the organic semiconductor nano lines can be formed simultaneously on the surface of the metal substrate with a large area.