Based on Tauc relation produced by energy conservation, one model with three parameters is developed to determinate band gap, Eg, by optical spectroscopy more precisely and reliably. Band gap, the index n stand for the nature and useful diagnostic A of material appear as parameter in this natural method. Thus, when non-linear fitting in the direction of least-square is finished, these key label Eg, n and A can be obtained accurately and reliably. Application results of model described above demonstrate that this method is most accurate, easiest and fast for energy band gap determination. Hence, this work makes the prediction of band gap and index of materials nature of materials more precise and reliable.