The glasses of the composition [xCoO–(29.5 −x)Na2O–55B2O3–15ZnO–0.5Er2O3] (where x=0, 0.3, 0.5, 1 and 3 mol%) have been prepared using melt quenching technique. The recorded UV/VIS/NIR absorption spectra have been studied and the resulting data was used for the ligand feld theory analysis, from which the crystal feld energy (10Dq), Racah parameters (B, C) and nephelauxetic ratio (β) were evaluated. The observed variations in the band gap and Urbach energy with CoO content have been discussed in relation with the glass structure. FT-IR spectra of the samples exhibit modes of vibration corresponding to the stretching of BO3 and BO4 unit. From IR studies, the number of non-bridging oxygens was increased, leading to loosen structure at higher CoO concentrations. The ESR spectra revealed that Co2+ ions are located both in distorted tetrahedral and octahedral sites.