Polymer composites have highly gained attention because of their high specific strength. On the other hand FRP is also widespread used in chemical plant as its high chemical resistant and easy to construction such as chemical storage tank.?However an accidental failure of top roof of chemical tank was reported after being used less than its expected life time 10-25 years. Generally many researchers reported that materials under liquid phase may show more severe condition than vapor phase. However those accidents were contrary to expectation from general knowledge. Therefore comprehensive study on the material durability over a wide range of condition is required.This study aimed to study degradation behavior of neat unsaturated polyester by non-chemical solution and chemical solution under isothermal and cyclic solution temperature on both liquid and vapor phase. Moreover a new design of apparatus to control a solution temperature has been developed. A cyclic solution temperature was cooling/heating as 40C/80C and stable at 80C for isothermal condition.The test results by using water and HCl showed flexural strength under vapor phase decreased about 25-30% by cyclic condition but only 10-20% by isothermal condition. However flexural strength under liquid phase decreased only 10% by water and 20-25% by acid solutions from both testing conditions. Surprisingly flexural strength of UP under vapor phase via cyclic condition by water was decreased almost as same as by acid solutions.The SEM morphology of tested specimen’s cross-section under cyclic temperature in vapor phase by SEM showed a layer which microvoids and cracks can be observed. Microvoids were also increased with increasing testing time which crack can easily propagate at that area. This would be pointed out that absorption/desorption by heating/cooling solution in vapor phase can weaken mechanical properties and lead to a failure of chemical tank.