Polaron tunneling is studied in xPA/nPPP/xPA (polyacetylene (PA), poly(p-phenylene) (PPP)) triblock copolymer, which has a well-barrier-well structure. The chain is modeled by a modified tight-binding Hamiltonian with an external electric field. It is found that there exists a threshold value of the external field for the polaron to tunnel through the middle barrier. The critical tunneling electric field mainly depends upon the length of the PPP segment. (C) 2003 Elsevier Science B.V. All rights reserved.