A field experiment was conducted in research farm of Isfahan University of technology, Isfahan, Iran in 2011 in order to evaluate the effects of various weeding periods on determination of critical period of weed control under drip tape and furrow irrigation systems. A split plot arrangement was used in a randomized complete block design with three replications. Irrigation treatments were accommodated in main plots and periods of weed control in sub plots. Weed treatments included two series of periods of weedless and weed interference at intervals of 20, 30, 40, 50 and 60 days after emergence. Two treatments of weedless and weed interference applied over the entire growing period were also considered as control treatments. Results showed that weed interference led to significant losses in plant height, number of branches per plant, stem and leaf dry matter and the yield of potato plants. The yield losses were different under the two irrigation systems. More yield losses were observed under drip than under furrow irrigation in weed interference treatments, while higher yields were obtained under drip irrigation in weedless plots. The period of weed interference had significant effects on weed density. The highest weed density was observed at early stage of growing period under both irrigation systems. Howevere, during the later stages weed density was reduced, due to self-weeding and intraspecific competition, but weed dry weight was increased. Overall, weed density was higher under drip compared to furrow irrigation. Effect of irrigation system was significant on yield and tuber size ditribution. The highest yield losses due to the production of undersize tubers under furrow irrigation were observed in weed interference treatments applied during early growing period but these yield losses under drip irrigation were obtained in weed interference treatments imposed during the later stages of growing period. The number of bigger tubers was increased in weedless treatments with an extention of growing period and the increase was more pronounced under drip irrigation. The beginning and the end of the critical period of weed control were determined based on 10 percent acceptable yield losses using logistic and Gampertz equations. The beginning of this period corresponded to the accumulation of 590 growing-degree days (GDD) under furrow irrigation, but under drip irrigation it occurred when 480 GDD was accumulated; i.e. 32 and 25 days after emergence of potato plants, respectively. The end of the critical period of weed control was occurred after accumulation of 1190 GDD under furrow irrigation while under drip irrigation it was achived when 980 GDD was accumulated; i.e. 59 and 50 days after emergence, respectively. Accourding to the results obtained in this study, weeds should be controlled within 25 to 50 and 32 to 59 days after emergence of potato plants under drip and furrow irrigation systems, respectively, in order to prevent substantial yield losses.