The research titled "Utilization of Reservoir Sediment of Gajah Mungkur For Mixture of planting Medium of coastal Sandy Soil in Cultivation of Lettuce (Lactuca Sativa L)". This research was conducted at Experimental Field of Faculty of Agriculture University of Muhammadiyah Yogyakarta from October to November 2016. The experiment was conducted by experimental method arranged in complete randomized design, with 5 treatments. The treatment were 9.375 kg of coastal Sand, 9.375kg of sediment, a combination of (2.34kg of coastal Sand and 7.03kg of sediment); (4.68Kg of coastal Sand and 4.68kg of sediment); (7.03kg of coastal and 2.34 kg of sediment). Each treatment was repeated 3 times, and each replication consisted of 3 plants, so there were 45 experimental units. The results showed that the sediment of Gajah Mungkur Reservoir can serve as a mixture of planting medium coastal soil for cultivation of lettuce. Treadment of 4.68 kg of coastal sand and 4.68 kg sedimen of Gajah Mungkur reservoir has the best effect to growth and lettuce result.