This study was conducted to determine the effect of variable nitrogen supply on yield parameters of two sugar beet varieties, as well as on biomass allocation to different parts of sugar beet plant at Safi-Abad Agric. Res. Center during 2003-4 and 2004-5. Five nitrogen rates (0, 60, 120, 180 and 240 kgha-1) and two monogerm sugar beet varieties (Shirin and Rasol) were arranged as factorial in a randomized complete block design with four replications. There was a significant difference between years for most sugar beet yield parameters. In the first year, nitrogen had no significant effect on root yield (mean of five rates was 93 t/ha) but changed sugar content significantly. N0 had the highest sugar content (13.7%). In the second year, root yield was influenced significantly by nitrogen so that N240 produced the highest root yield (80.9 tha-1) whereas sugar content did not change significantly. There were no significant differences between two varieties for all yield parameters in the two years. Increasing N shifted allocation of biomass to petiole and crown and reduced its allocation to root. But no differences between nitrogen rates for biomass allocation to leaf were found. All nitrogen rates had same leaf number, leaf area index and canopy closure during the first year, but during the second year increasing nitrogen levels accelerated canopy closure and increased leaf number and leaf area index. Nitrogen had no significant effect on nitrogen content and nitrogen uptake of different plant parts in the first year. Total nitrogen uptake by sugar beet at the end of season was 574 kgha-1 in the first year. In the second year, total nitrogen uptake was influenced by application of nitrogen, so that uptakes in 0 and 240 kgha-1 N were 186 and 351 kgha-1 respectively.