Here we demonstrate a simple and effective method to fabricate graphene ribbon with a sub-micrometer width down to 260 nm by using an electrospun polymer nanofiber as a physical etch mask. Our method involves electrospinning polystyrene nanofiber onto chemical vapor deposition-grown graphene film, followed the oxygen plasma etching to remove the exposed graphene without disturbing the underlying graphene. This work shows that the width of the resulting graphene ribbons can be engineered by controlling the nanofiber size and etch conditions. Based on this graphene ribbon, we fabricated a graphene-field effect transistor with a bottom-gated geometry, which shows an ambipolar characteristic with a hole and electron mobility of 1636 and 134 cm(2)/(V s), respectively. Our approach here may allow the fabrication of sub-micrometer graphene ribbon for graphene-based electronics.