The role of BEG 1,2-butanediol as a reducing agent in Co-based nanoparticle synthesis in the polyol process has not been well-detailed yet. So, we focused on the determination of the main active species derived from 1,2-butanediol (BEG) in Co-based nanoparticle synthesis and their reducing abilities through density functional theory (DFT) calculations. In the reaction medium, BEG is deprotonated by the hydroxyl ions introduced in the solution then oxidized by the metal ions. The progression of reduction and dissociation reactions of metal ions is relatively related to the reducing ability of polyols. Three species which are: dianion, monoanion and neutral molecule of BEG were considered in our investigation. The highest occupied orbital energy was estimated for the different configurations. Considering the experimental and theoretical studies, themonoanion state was suggested as the most active form. A comparative study was carried out between three polyols: BEG, PEG (1, 2-propanediol) and EG (Ethylen glycol), which are the most used solvents in Co-based nanoparticle synthesis.We showed that the highest occupied orbital energy of BEG monoanion state is relatively high compared to PEG and EG ones. Thus, BEG could reduce metal ions more easily by giving its electrons and its use can make the reaction kinetics faster