We theoretically Study the translocation time and the mean velocity of a long polymer threading a long nanopore oil an external electric field. Theoretical method, which explicitly takes into account the nucleation theory, is presented based on the built polymer expanded model. We overcame the previous theory's defect which did not consider the real length of the pore. The present model implies that the length scale of a pore plays a very important role in the process of polymer translocation. Our calculation results are in agreement with those of previous experiments. (c) 2006 Elsevier Inc. All rights reserved.