A solution-processable electrochromic (EC) polymer, poly(4,4-dioctyl- cyclopenta[2,1-6:3,4-b′]-dithiophene) (PDOCPDT) is prepared by means of chemical oxidative polymerization of the corresponding monomer. The UV-vis spectrum of the spin-coated PDOCPDT film displays an absorption maximum of 580 nm. Although the polymer is deep blue in its neutral state, it turns to transparent bluish after being oxidized. PDOCPDT film (thickness 120 nm) exhibits high coloration efficiency (CE)-as high as 932 Ccm-2 at 580 nm, low response time (0.75 s), high optical density (0.75 at 580 nm), and high-level stability for long term switch (it switches repetitively 1000 times with less than 8 % contrast loss). The electrochemical stability and redox potentials of PDOCPDT films are independent of film thickness (50-180 nm) and active area (up to 2 cm × 2 cm). Nevertheless, optical contrast increases as the film thickness increases, although the CE and response time changes irregularly with the film thickness. The good EC properties combined with the easy film fabrication process make PDOCPDT a notable candidate for application in EC devices. (ECDs) A simple transmissive-type BCD with good CE using PDOCPDT film as an active layer is also demonstrated.