A series of all-conjugated diblock copoly(3-alkylthiophene)s with varying alkyl chain length, poly(3-butylthiophene)-b-poly(3-hexylthiophcne) (P3BT-P3HT), poly(3-butylthiophene)-b-poly(3-dodecyl-thiophene) (P3BT-P3DDT), and poly(3-hexylthiophene)-b-poly(3-dodecylthiophene) (P3HT-P3DDT), were synthesized by modified Grignard metathesis polymerization. The obtained diblock copoly(3-alkylthiophene)s have well-controlled block ratios and molecular weights with narrow polydispersity indices. The diblock copoly(3-alkylthiophene)s comprised by the blocks with the alkyl side-chain length different by two carbon atoms (represented by P3BT-P3HT) have the ability of cocrystallizing into an uniform crystal domain, with the mutual interdigitation of the different side chains of the two blocks into a common interchain lamella. Other diblock copoly(3-alkylthiophene)s with the side chains different by more than two carbon atoms prefer to microphase separating into two crystal domains formed by the independent crystallization of each block. The A FM images revealed the characteristics of microphase-separated morphology in P3BT-P3DDT and P3HT-P3DDT diblock copoly(3-alkylthiophene)s.