To assess the influence of achiral unidentate ligands on electronic circular dichroism spectra, the excitation energies, oscillator and rotational strengths for a series of trans-bis-ethyl-enediamine and propylenediamine cobalt(Ⅲ) complexes containing the unidentate ligands X (X= Cl- ,CN- ,NH3,NO-2 ,N-3 ,etc) have been calculated at the TDDFT level using the B3LYP functional and 6-311++G(2d,p) basis set. It was found that, for the chelates with nonaxially symmetry unidentate ligands(X=N-3 ,SCN- ,NO-2 ), the chiral orientations(δ/λ) of the N-3 and SCN- dominate the ECD spectra. However, the contribution from the last one ligand is rela- tively smaller than that of the first two. Such phenomenon has been successfully observed in the experimental ECD spectra. The δ-orientation of NO-2 exhibits one strong negative and another weak positive band shapes in the ECD curves, and such distributions is consistent with the experimental observation of trans-[Co(pn)2(NO2)2]+. For all of the chelates, the ECD patterns in the long-wave length region are dependent on the spectrochemical series of the unidentate ligands X in the sequence from weak to strong: SCN- ,Cl ,N- 3 ,H2O,NCS- ,NH3,en,NO-2 ,CN- .The first absorption peak of the ECD spectra is negative in sign for the chelates with stronger unidentate ligands, while it is positive in sign for the complexes with weaker unidentate ligands.