When furnace 1 and 2 were shutdown, we did in situ metallography on furnace tube randomly to know its reliability. Tube which damage a failed tube was taken to laboratory to be analyed the failure cuased to avoid the same failure in the future. The examination and testing held on tube furnace 1 and 2 CDU III was : fractography and metallography examination, hardness testing, chemical composition analysis and SEM – EDX examination. From the examination and testing can be concluded that the tube furnace can not resist over heating as a result of deposit , graphite was formed in outer side and sulfide corrosion was formed in inner side causing diameter thinning. Depletion and graphitization region is a critical area so if the tube receives a static load and the excess temperature the tube will damage.