A pot experiment was conducted to find out the effects of nitrogen content of irrigation water and soil EC on lettuce growth under plastic film house conditions. The square-pots with 42 x 54.5 x 22 cm in length, width and height, were filled with two different soils of different EC. Lettuce was grown with different nitrogen fertilizer application levels including non fertilization (Non-F), decrement of 50% of nitrogen fertilizer recommended by soil testing (DNFRST-50) and fertilization recommended by soil testing (FRST). Two kinds of irrigation water of different nitrogen contents, $6.6mg\;L^{-1}$ and $21.0mg\;L^{-1}$, were used for the experiment. In the low EC soil irrigated with low nitrogen water, fresh weights of lettuce were 6,733, 11,933 and $12,733kg\;ha^{-1}$ for the treatments of Non-F, DNFRST-50 and FRST, respectively. While with high nitrogen water, the yields were 9,733, 13,400 and $12,800kg\;ha^{-1}$, respectively. In the high EC soil irrigated with low nitrogen water, lettuce yields of the Non-F, DNFRST-50 and FRST treatments were 12,400, 12,867 and $10,400kg\;ha^{-1}$, respectively, and with high nitrogen irrigated water lettuce yields of the Non-F, DNFRST-50 and FRST treatments were 13,600, 14,067 and $10,733kg\;ha^{-1}$, respectively. Nitrogen uptake of lettuce from ferilizer in DNFRST-50 was higher than of FRST. Nitrogen uptake of lettuce from irrgation water was found in soils of low EC, but it was not found in soils of high EC. These results suggest that both soil EC and nitrogen content of irrigation water should be considered when we recommend the level of fertilizer application for lettuce.