Among Ziegler-Natta catalysts used for 1,3-butadiene (1,3-BD) polymerization, the advantage of a neodymium (Nd)-based catalyst is that it provides butadiene rubber (BR) with a high content of cis-1,4 configuration and a low amount of vinyl-1,2 units. Whereas, a cobalt (Co)-based catalyst can produce BR with a low content of trans-1,4 configuration. Thus, this research was aimed to prepare BR containing a high content of cis-1,4 configuration with low amounts of both trans-1,4 and vinyl-1,2 units using a combination of Nd- and Co-based Ziegler/Natta catalysts with triethyl aluminum (TEAL) and diethyl aluminum chloride (DEAC) acting as a co-catalyst and a chlorinating agent, respectively. The effects of the molar Co/Nd ratio, TEAL concentration, DEAC loading, 1,3-BD content, solvent type, and reaction temperature on %conversion, microstructures, molecular weight, and molecular weight distribution of the obtained BR (Co/Nd-BR) were evaluated. The Co/Nd-BR having >97% of cis-1,4 configuration, <2% of trans-1,4 structure, and <1% of vinyl-1,2 unit with >80% conversion was achieved when 3.01 M of 1,3-BD concentration was treated in a toluene/cyclohexane mixture (7/3 [w/w]). The Co/Nd-BR exhibited no gel formation with high mechanical performance, which was equivalent to commercial BR produced from a Nd-based catalyst system. POLYM. ENG. SCI., 55:14-21, 2015. (c) 2014 Society of Plastics Engineers