This paper aims to study the lesson learned from the Mox plant case between Ireland and England. For this case, it reflects that the International Tribunal for the Law of the Sea hereinafter ITLOS’s approach of protection the marine environment under the United Nation Convention for the Law of the Sea and can be applied for the Caesiam-137 case in Thailand. The results show that in the Mox plant case, it can show and clarify the approach of prescription for provisional measure of tribunal as follows: the precautionary measures are laid down by ITLOS, the tribunal shall be considered appropriate under the circumstance to prevent serious harm to the marine environment. For this case, the tribunal prescribed the provisional measure because the Mox plant may release the radioactive material to the Irish Sea and the aforementioned cause affected the marine environment. Tribunal applies precautionary principle and takes the provisional measure to protect the environment. Additionally, these approaches of prescription of measures should be implemented by the Thai authority, Department of Industrial Works for the Caesium-137 case also. Although this Caesium case, from the inspection of state officials find that the nearby area of iron smelting plant is still safe in regard the well-being and environment. The authority should forecast the risks and effects that will happen from Caesium case by taken measures on precautionary principle as in the Mox plant case.