Kramers-Kronig dispersion relations are the inherent cross-constraints on the real and imaginary parts of the complex refractive index of dielectric materials. These relations are utilized in this study for estimating leaf refractive index from leaf absorption spectra. A mixing model is developed for calculating the leaf refractive index with respect to the variation of leaf mass compositions and absorptive properties. The model is based on the variational function fitting method of Kramers-Kronig analysis. The validity of the model is analyzed with pure liquid water measurements and well-established leaf optical measurements. It is found that the model has a very good performance in modeling the spectral details of leaf refractive index and provides an efficient physical framework for improving and extending the leaf-level optical property model.