Estimation of optimum fertilizer rates is needed because of growing economic and environmental concerns. Optimum fertilizer rates can be determined by fitting statistical MODELs to yield data collected from N fertilizer experiments. The main goal of this research was to compare and evaluate quadratic, square root, Mitscherlich, rectangular hyperbola, linear plus plateau and quadratic plus plateau MODELs for describing the response of sugar beet to N fertilizer. Data used were obtained from a furrow irrigation system experiment with five N fertilizer rates: zero (control), 60, 120, 180, and 240 N kgha-1 with three replications in Ekbatan Research Station, Hamedan, Iran, during 2003 and 2004. Economic optimum N fertilizer rates were obtained based on fertilizer and sugar beet price during 2003 and 2004. Economic, optimum N fertilizer rates varied depending on the fertilizer to crop price ratio and MODELs used. Results of this research showed the quadratic MODEL described the yield responses and economic, optimum N fertilizer rate in sugar beet cultivation better than the other MODELs. Economic, optimum N fertilizer rates due to this MODEL were 235.8 and 248.9 kgha-1 in 2001 and 2002, respectively. Economic optimum N fertilizer rates based on N fertilizer subsidy and non-subsidy prices were 234.7 and 225.1 kgha-1 for 2003 MODEL, and 247.9 and 240.8 kgha-1 for 2004 MODEL, respectively.