The piping system is very important, the function of piping system is to flow fluid to one poin to another. However, overstress is possible in designing of piping system. One of them is the piping system connecting RE-101 in VCM plant at Asahimas Chemical company because a large diameter up to 36". The purpose of this design is to determine the maximum of allowable stress and deflection in the piping system in order to avoid excessive stress and deflection which can cause the disruptive operation of the piping system. This designing is conducted by collected required data from Asahimas Chemical company then make a model on CAESAR II 2016. The calculation result that has been done by using software is analyzed with compare the maximum stress and deflection must not exceed than allowable stress and deflection. If the ratio more than 100%, the piping system is needed to redesign by modify support that already exist or adding new support on certain location. The initial design of the piping system shows over than 99% and 98,7% stress ratio at 1st and 10th loadcase. All of maximum deflection values that occurs in every loadcase is exceed, so that it is required to redesign the piping system. The redesign that has been done were added 9 new support at certain location and modifiy 3 existing support. After modification on the piping system, there was a decrease stress ratio on loadcase 1 and 10 to 29.5% and 89.7%. The deflection value also decreases in each loadcase so that nothing exceeds the allowable deflection. Therefore the piping system connecting RE-101 at VCM plant on Asahimas Chemical company can be categorized as good and safe.