With the objective to remove buckhorn (Plantago lanceolata L.) and rumex (Rumex spp.) seeds from birdsfoot trefoil (Lotus corniculatus L.) seeds a spiral separator was adapted with following basics modifications: 1) spiral with 5,5; 6,5 e 7,5cm in width; 2) step (distance between two consecutives lap of the spiral) with 13, 14 and 15cm; 3) feeder with 6,0mm diameter openning allowing a feed rate per spiral of 10kg/hr for birdsfoot trefoil. For yield and efficiency determination of the separator, samples were used with equal wheight as for the exam of noxious weed seeds, being each sample contaminated with 60 seeds of buckhorn and 60 seeds of rumex. The yield was determined on the bases of birdsfoot trefoil seed loss percentage while the efficiency based on the removel percentage of buckhorn and rumex seeds. The results showed that the best combination is the spiral with 7,5cm in width and 15cm in step, obtaining an efficiency higher than 95% and yield above 93%. It is suggested that a spiral separator be included in a seed conditionning une for birdsfoot trefoil seeds due to its efficiency and yield, easy fabrication, low cost and easy maintenancy.