Zigzag-shaped [6]phenacene isomers with fused thiophene rings, fulmineno[2,1-b:10,9-b']dithiophene (FuDT-α) and fulmineno[1,2-b:9,10-b']dithiophene (FuDT-β), were syntheized and their p-type organic semicondutor properties were studied. Small effective masses of holes were estimated from the crystal structures of both isomers, which was particularly demonstrated by the hole mobility of 10.5 cm2 V-1 s-1 for FuDT-α single crystals.