A new series of conjugated polymers constituted of 3-alkylthiophenes and 1,4-di(1,3,4-oxadiazolyl)phenylene are synthesized through precursor polymers of polyhydrazides. The solubility of the precursor polymers and the corresponding target polymers is improved by increasing the length of side chains on thiophene rings, which results in a higher degree of the conversion from the precursors to the final polymers and thus results in longer effective conjugated length in the polymers bearing longer side chains. The thermal stability and electrochemical properties of the polymers are studied. The polymers emit blue light under the irradiation of UV light in their film states. The relative fluorescence yield in solutions increases with the increase in length of side chains on thiophene rings in the polymers and reaches up to 79% for the polymer bearing octyl side chains. The possibility of polymers being used as electroluminescent material in polymer light-emitting diodes (PLEDs) is demonstrated by the successful fabrication of a single-layer device of indium tin oxide (ITO)/polymer/Ca, which emits visible blue light above 8 V of forward bias.