Calculation of Temperature and Species mass fractions is one of the most important concerns in solving combusting flows. In laminar diffusion flames, in order to calculate the exact values of Temperature and Species mass fractions, it is usual to define a parameter named mixture fraction which indicate the ratio of fuel mass to total fuel and oxidizer. In turbulent diffusion flames in addition to the mean mixture fraction, the mixture fraction variance also plays an important role in predicting the Temperature and Species mass fractions. Therefore finding appropriate models and equations to simulate these two parameters would be really helpful. In turbulent diffusion flames, important parameters such as velocity, temperature and species mass fractions act like random parameters. Study of these parameters requires appropriate averaging methods such as the Favre averaging method. In the turbulent regime, temperature and species mass fractions are functions of mean mixture fraction and its variance. The transport equation of mixture fraction variance includes terms representing the production and the dissipation of mixture fraction variance. The dissipation term in this equation needs modeling. There are two different ways to consider the effects of scalar dissipation rate. The first one is to develop a traort equation for this parameter. In order to write such equation many of scientists have tried to find the best coefficients for the equation. This method is very complicated because there are many terms in this equation. Therefore it is better to find another way to solve the problem.In the field of modelling there are two categories of sub-models. The first one which is called the Equal Scales model is based on the hypothesis that time and length scales of scalar are equal to time and length scales of fluid flow field. This model is one of the most applicable models and for years it has been used to calculate the scalar dissipation rate. The second one is the Non-Equal Scales model.In this model these scales are not necessarily equal.In the present study, these two models with different coefficients have been used to calculate the mixture fraction variance in three diffusion flames and the best coefficients for each flame have been presented. Besides, for the first time, a new model is proposed which is derived from the traort equation of mixture fraction dissipation rate. In this model,it is assumed that the diffusion and convection terms in the traort equation of the scalar dissipation, cancel out each other, therefore, the method is very similar to the algebraic Reynolds stress model.The results from this model have more accordance with the experimental data in comparison with the other models. Key words : Favre Averaging, Mean Mixture Fraction, Mixture Fraction Variance, Scalar dissipation rate, Diffusion, Equal Scales, Non-Equal Scales