Arbuscular mycorrhizal fungi are a group of fungi that can have mutualism symbiosis with more than 80% of higher plants. They are reported many advantages on plant cultivation. Previous research suggested addition of 50 spores of Glomus mosseae per plant was the best condition in improving butterhead lettuce growth in pot experiment. This research was performed to present the use of this condition in trial field under organic farming system.  Fifty spores of G. mosseae were added in soil before planting butterhead lettuce seedling. Growth parameters including leaf number, leaf and root fresh weight, and leaf width and length as well as percentage of root colonization on day 15, 30, 45 and 60 after inoculation comparing with non-inoculation as a control were measured. Leaf number of AMF treatment was significantly higher than control at day 30. Leaf and root fresh weight also presented growth enhancement in AMF inoculation treatment on day 30. AMF inoculation did not affect leaf size. The correlation analysis between root colonization and each growth parameter ensured that an improvement of plant growth was the result from the symbiosis of AMF in plant root.