The using of a reliable and accurate new method (called in literature as derivation of absorption spectrum fitting (DASF)) for evaluation of the optical band gap (E-g) and also the exact nature of charge carriers optical transitions, is investigated in ZnSe and ZnSe: Cu nanocrystals (NCs) synthesized by rapid microwave irradiation. This method can be performed by using the output of UVeVisible spectroscopy. The obtained E-g values are within the range of 2.985-3.261 eV, depending to the microwave irradiation time and Cu dopant percentage (decreasing trend with increasing of irradiation time and Cu content). The DASF-based obtained results for ZnSe and ZnSe: Cu nanoparticles, showed the more precise values of band gap, with the same trend of previously qualitative reported data on the same samples. Also, the direct gap nature of their optical transitions was justified. To perform the method, there is no any need to the concentration of solutions and merely one need the direct absorption or transmission spectra. In other word, DASF technique was employed on ZnSe NCs to confirm its validity and to avoid non-precise reports on optical band gap which can affect on the device optimizations based on these samples. Moreover, using the values of E-g, refractive index and dielectric constant of each sample were obtained at the absorption edge. Also, the width of the tailing states in the gap (Urbach energy: ETail) was estimated and were within the range of 0.049-0.122 eV, which their very small values in compare with E-g imply to the sharp valence and conduction band edges; it means the good crystallinity nature of the produced samples. (C) 2016 Elsevier B. V. All rights reserved.