Ulva lactuca is a green macro alga called sea lettuce, was isolated from marine environment of Egypt. Its phytochemical constituents were determined by quantitative and qualitative tests using nine solvents different in their polarity. Its chemical structure also was proved by FT-IR spectroscopy. According to its enrichment chemical structure its biological activities as antioxidant, bactericide, fungicide and insecticide against human, plants, fish, poultry and animals pathogens and insects were investigated. Aqueous extract was the best as bactericide and fungicide and also give a good results as insecticide.  Ulva lactuca showed antioxidant activity with IC₅₀ = 9.6 mg/ml. Biological activity of Ulva lactuca related to its chemical structure and revealed its importance as natural pesticides.