This experiment was conducted to determine the effects of defoliation intensity at different sowing dates on quantitative and quality characteristics of sugar beet at Motahari Research Field Staiton of Sugar Beet Seed Institute, Karaj, Iran, in 2010 and 2011. The experiment was arrangemed as split plot using randomized complete block design with four replications. Main plot were assigned of two levels of sowing date (sowing on time, in early spring, and late, about one month later) and subplots consisted of combination of four growth stages, initial (germination till six-leaf), canopy development (six-leaf till canopy closure), mid-season (canopy closure) and late season (reduction of canopy closure till harvest time), with four levels of defoliation intensity (25, 50, 75 and 100 percent of leaf blade). In each main plot, four treatments including the control treatment (without defoliation) and three sowing dates, simultaneous with defoliation at the first three stages of defoliation times, were considered. After harvesting, root yield and other main quality properties was measured. Results showed that root and white sugar yields with a delay of about 26 days reduced by 11 and 7 percent, respectively. Late season sever defoliation reduced sugar content, white sugar content and extraction coefficient of sugar beet. Defoliation in mid-season and late stages of crop life-cycle significantly reduced sugar yield. It was concluded that when leaves of sugar beet are damaged, by any mean, re-sowing is not recommended and the filed could be preserved.