Abstract: In order to investigate tuber size and nitrogen fertilizer on nitrogen uptake and nitrate accumulation in potato tuber cultivar Agria, a factorial experiment based on randomized complete block design with three replications was carried out in Ardabil, Iran, in 2006. Factors were nitrogen fertilizer level (0, 80, 160 and 200 kg ha-1 net nitrogen) and tuber size (<40 = small, 40-80 = medium and >80 = large, g). Results showed that the most tuber yield, No. of tuber per plant, mean tuber weight and tuber dry weight were resulted at medium tuber size. Also, the most tuber yield, mean tuber weight, tuber dry weight and tuber nitrogen percent were observed at 160 kg ha-1 nitrogen. The most nitrogen taken up in tuber and aerial parts and nitrate accumulation in fresh and dry weight was gained at 200 kg ha-1 nitrogen and medium size. The most important result from this study was that nitrogen application over the favorite values, resulted in reduction of crop production along with increasing nitrate accumulation in tubers. So, nitrogen value of 160 kg ha-1 and medium tuber size to get the highest yield and suitable planting and eating usages are recommended, respectively.