This study was conducted in order to determine the effects of beneficial microorganisms on two lettuce cultivars ('Maritima' and 'Patagonia') in organic production. Two beneficial microorganisms, namely mycorrhiza (Glomus spp.) and Trichoderma harzianum stain KRL-AG2 and their combination (mycorrhiza + T. harzianum) were tested comparing with the control (no treatment). Lettuce (Lactuca sativa L.) plants were grown between 14.11.2016 and 02.03.2017 in an unheated greenhouse with a plant density of 5.7 plant m. At the end of the growing period, yield and head quality parameters such lettuce size, color, vitamin C and nitrate contents were determined. The results showed that cultivar 'Patagonia' gave the highest yield (2.18 kg m) and highest head size (366.16 g m), however among the tested microorganisms, mycorrhiza had higher performance as beneficial microorganism compared with the other treatments.