This study aims to evaluate the effect of solar UV radiation on defensive enzymes activities in lettuce seedlings. Seeds of lettuce were grown in high tunnels for 35 days, exposed to three different levels of solar UV, created by using three different types of plastic films. Each plastic film transmits different levels of solar UV (0% in UV-B, 4% in UV-L and 83% in UV-T). The obtained results indicated that solar UV radiation has led to significant decrease in seedling fresh weight. UV-B treatment resulted the highest shoot and root length while UV-T treatments exhibited the highest shoot: root ratio. Solar UV radiations have no effect on peroxidase activity in shoots. While ascorbate peroxidase was activated and catalase was inhibited in shoot by UV-T treatments. In lettuce roots, peroxidase, ascorbate peroxidase, and catalase activities were increased by increasing the level of solar UV radiation. In case of UV-T, phenylalanine ammonia lyase (PAL) activity in both shoots and roots of lettuce seedlings was higher than those of other treatments. In addition, Phenols are accumulated in lettuce shoots as a result of UV radiation in UV-L and UV-T treatments. Also, phenols in roots increased by increasing solar UV dose. The study concluded that solar UV radiation induced some antioxidant enzymes, increased the accumulation and biosynthesis of phenolic compounds and reduced lettuce seedlings growth.