In this paper, a new mathematical model has been presented to determine the equilibrium segregation (k 0 ) and effective segregation (k eff ) coefficients for neodymium (Nd) in YAG crystal grown by Czochralski (CZ) method. Determination of diffusion coefficient (D L ) of Nd impurity in molten YAG is also investigated. In this model, utilizing Lambert W-function is a new idea to solve the Scheil equation for calculation of effective segregation coefficient. The Nd concentration in the crystal has been measured by optical absorption method to calculate k eff . The analyses show that the k eff is related to the growth parameters such as crystal growth rate (u g ) and crystal rotation rate (ω), (keff~ug/ω) but it is independent of the Nd concentration in the initial melt (C 0 ). Based on obtained k eff and experimental growth data, k 0 and D L of Nd in molten YAG have been calculated. For all experiments, the average value of k 0 =0.216 and D L =1.4×10 −6 (cm 2 /s) are obtained. Our results are corroborated by the theoretical and experimental data from the literature.