Sunscreen can prevent human from skin cancer. Numberous research found UV rays is an important factor in the cancers incidences, Especially the most risky Uv radiation(UVA) 320 nm to 400nm. UVA can penetrate deep dermis/induce cutaneous photo damage, inframation ,skindarkening ,receiving for long time, can affect to DNA damage that cause of skin cancer. Sunscreen consist of a metal compound such as iron, chromium, Zinc ,Titanium, etc. There are import protective UV ray likely physical barrier block on skin, Metal compound in sunscreen can block UVA, So UVA can’t harm human’s skin.