Two conjugated polymers (CPs) P-tCzC12 and P-tCzC16 comprising alternating dithieno[3,2-b:6,7-b]carbazole and 4,4'-dihexadecyl-2,2'-bithiophene units have been designed and synthesized. Upon thermal annealing, they can form ordered thin films in which the polymer backbones dominantly adopted an edge-on orientation respective to the substrate with a lamellar spacing of 24 angstrom and a p-stacking distance of 3.7 angstrom. Organic thin-film transistors (OTFTs) were fabricated by solution casting. A hole mobility of 0.39 cm2 V-1s-1 has been demonstrated with P-tCzC16. This value is the highest among the CPs containing heteroacenes larger than 4 rings.