We systematically investigated the tuning of the reduction potentials (E0 ) for the first row transition metal (M=Cr, Mn, Fe, Co, Ni) complexes with the functionalized 1,4,7-Triazacyclononane-N,N',N”-triacetate (TCTA) ligands by means of DFT calculations. To predict reliable E0 , the modified UB3LYP functional and equilibrium concept between high and low spin states were utilized. The functional groups NH2, CN, −F, and NO2 were attached to the carbon atoms carefully selected by considering the LUMO and steric hindrance. Based on firmed accuracy of DFT calculations, finally we obtained the calculated E0 for a series of complexes. It was found that electron donating group such as NH2 will cause a negative shift of E0 while electron withdrawing groups have the opposite effect. The overall trend of the calculated E0 s according to ligand modifications were predicted to have the order as −NH2 < Pristine <−F≈−CN <−NO2. In addition, optimized geometries, LUMO, vertical electron attachment and energy components constituting E0 were discussed in detail to assist the further understanding for E0 s. Consequently, we suggested that 16 complexes can play a role as an electrolyte in aqueous redox flow battery. They can be classified into 5 groups having similar E0 ranges: Group I (- 0.6V∼− 0.7V), Group II (around 0.0V), Group III (around 0.3V), Group IV (0.6∼0.8V) and Group V (1.1∼1.2V). Especially, it is expected that [MnLF], [MnLCN] and [NiLNH2] can be used as promising catholyte candidates possibly possessing high E0 s which almost reach to the reduction potential limitation 1.25V in aqueous redox flow battery. Our systematic approach to tune E0 can be applied to the design of other complexes via rational ligand modification.