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Objective: To determine the wash resistance of Olyset? Plus using World Health Or-ganization Pesticide Evaluation Scheme standard washing procedure and to assess the value of knock down and mortality rates of Anopheles stephensi at different regimens of long lasting insecticide treated nets washings. Methods: The study was conducted at the Bioassay Laboratory of Culicidae Insectary, School of Public Health, Tehran University of Medical Sciences, Iran. The net was made of polyester impregnated with permethrin and piperonyl butoxide at a ratio of 2:1. The washing resistance was assessed using Le Chat? soap and a shaker incubator set at a speed of 155 r/min, 30 ℃ for 10 min. The cone bioassay test was carried out according to World Health Organization recommended guideline with tolerant field strain of female Anopheles stephensi to pyrethroids. Results: The knockdown and mortality rates of female mosquitoes exposed to Olyset? Plus from un-washed nets to 2 washings were 79.7% and 88.8% respectively. Mortality was dropped to zero while active ingredient estimated 0.532 μg/100 cm2 to 0.481 μg/100 cm2 after 15 washings. A positive correlation was seen between residues of permethrin on nets, knockdown rate and mortality rate of female Anopheles stephensi exposed to different regimes of washed Olyset? Plus (r = 0.954, P = 0.001). Conclusions: It is recommended that a preliminary survey conducted on resistance level of Anopheles vectors before the distribution of Olyset? Plus in malaria endemic communities.