Western flower thrips, Frankliniella occidentalis(Perg.), is one of the major pests of lettuce, Lactuca sativa. To develop the economic thresholds for western flower thrips on lettuce, the thrips was inoculated into two types of lettuce, green deeply lobed leaf lettuce which was harvested every week and red frilled & crinkled leaf lettuce which was harvested one time, with a density of 0, 1, 3, 7, 10 thrips per one lettuce after planting 1 week and 3 weeks at the plastic house in 2006, and 0, 1, 3, 5, 7 thrips per one lettuce after planting 1 week in 2007. There was close correlation between the initial thrips density and the marketable quantity of lettuce : y = -155.36x + 3564.7, r2=0.9724('06), y = -167.82x + 3474.5, r2=0.8897('07) in the green deeply lobed leaf lettuce inoculated the thrips after planting 1 week, y = -109.65x + 3549.1, r2=0.9384('06) in the green deeply lobed leaf lettuce inoculated the thrips after planting 3 weeks, y = -197.09x + 3334.9, r2=0.9707('06), y = -244.08x + 3425.9, r2=0.8914('07) in the red frilled & crinkled leaf lettuce inoculated the thrips after planting 1 week, y = -101.07x + 3457.6, r2=0.7334('06) in the red frilled & crinkled leaf lettuce inoculated the thrips after planting 3 weeks. The economic threshold for western flower thrips on lettuce was ranged 0.20~0.30 thrips per one lettuce in the blue type lettuce and 0.14~0.33 thrips per one lettuce in the red type lettuce.