Ground cherry (Physalis divaricata L.) is one of the most important summer weeds in sugar beet crop in the west of Iran. In order to estimate the damage rate of this weed, field studies were conducted to quantify the effect of ground cherry density on sugar beet yield and to determine relationships among different weed densities (0, 0.5, 1, 2, 4, 8 and 16 plants m -2 ) and sugar beet yield in 2008. The experiments were arranged in a randomized complete block design (RCBD) with four replications. In addition, the neighborhood effect of ground cherry was assessed in a completely randomized design. Neighborhood effect was surveyed from zero to 125 cm apart from each sugar beet plant to ground cherry. Density of ground cherry was estimated as the systematic method in 30 sugar beet fields that were chosen randomly.Phonology of ground cherry was recorded based on the GDD (Growth Degree Day) and date. Results showed that two weed plants m 2 of this weed resulted in 34% damage to sugar beet. Ground cherry significantly reduced yield of sugar beet when sown 50 cm apart from crop plant. Crop damage of sugar beet was 41% when ground cherry seeds were sown at zero cm apart from each sugar beet plant. Flowering of ground cherry occurred in the middle of June when it received 61.45-75 GDD and it proved that ground cherry is a neutralized weed to the long day.