The aim of this study, was to determine the best non-linear model and measure interval (biweekly or monthly) in explaining the body weight-age relationship in Norduz male lambs born in 2004. For this aim, Brody, Logistic, Gompertz and Richards non-linear models were fitted to the average body weight-age data with 15 (biweekly) days and 30 (monthly) days of measure intervals. Although, Logistic model become equal to Richards model (99.8%) for two intervals, Logistic model had lower RMSE than Richards model. Therefore, the best non-linear model for 2 intervals was Logistic model having the highest coefficient of determination (R-2) but the lowest Root of Means Square Error (RMSE). Contrary to Brody non-linear model, the usage of 30 days of measure intervals performed positive effect on Logistic, Gompertz and Richards non-linear models instead of 15 days of measure interval. As a result, it was concluded that the best non-linear model for Norduz male lambs was Logistic model and the appropriate measure interval for Norduz male lambs was monthly interval.