We aimed to formulate and validate mathematical functions that can be used to determine cardinal temperatures and the effect of temperature on biological days required from seeding to emergence of wheat cv. ‘Tajan’. For this purpose, different non-linear regression models including flat, logistic, quadratic, cubic, dent-like, segmented and Curvilinear were used. Root mean square of errors, coefficient of determination and regression coefficients of predicted versus observed values were used to find the appropriate model. Investigating regression coefficients indicated that only in flat and logistic models, related coefficients were not significant. Other models (quadratic, cubic, dent-like, segmented and curvilinear) were not used to select the best model. Because the root mean square of errors was less in the logistic model than in the flat one, the former was chosen as the best model to describe the response of emergence rate to temperature. Using this function, base and optimum temperatures were 5.2 and 24.8°C. This function and its parameters can be used in wheat simulation models to predict seedling to emergence duration based on a thermal time concept. Also, required biological days from seeding to emergence using these models varied from 2.78 to 3.98. _

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