In head lettuce seedlings, NaCl stress was treated with hydroponic culture containing 0, 50, or with 100 mM NaCl in the seedling grown up to two leaf stages. Our focus was on the effect of NaCl on fresh and dry weights, antioxidant materials, and inorganic ion level. Fresh and dry weights of head lettuce seedlings increased with the increase in salinity while the optimal growth occured at 50 mM NaCl. The chlorophyll a (Chl a), total Chl and Chl a/b ratio increased 6 days after treatment with 100 mM NaCI. However, the Chl b content decreased. Total glutathione increased only in the root of head lettuce seedlings, whereas significant increase of total arcorbate content was observed in both shoot and root after the treatment with 100 mM NaCl. In addition, the NaCl treatment resulted in the decreased level of spermidine content, and a increased spermine content. Furthermore, $Na^+$ content in shoot and root increased significantly while $K^+$, $Ca^{2+}$, and $Mg^{2+}$ content decreased. The alteration of inorganic ion level after treatment with NaCl caused the reduction of $K^+/Na^+$, $Ca^{2+}/Na^+$, and $Mg^{2+}/Na^+$ ratio with the increase of NaCl concentration. Taken together, these findings indicate that the treatment of NaCl causes the induction of oxidative stress, and results in the alteration of metabolic mechanism in head lettuce seedlings.