The repeated failure of waste heat recovery boiler (WHRB) coupled with ammonia oxidation reactor in a fertilizer plant is investigated in this study. Failure was observed in the boiler shell where thickness was 14mm constructed with 1Cr-0.5Mo (SA 204 Grade A) steel. After 289080 hours operation a failure was observed at the upper vessel section and working at temperature 500 - 600^oC and internal pressure of 5.5kg/cm². Air/ammonia mixture was being fed in the reactor coupled with WHRB and as a result of the exothermic reaction of ammonia oxidation the temperature was raised to 800^oC in reactor. To recover this heat a steam was being introduced at a pressure and temperature of 42kg/cm² and 135⁰C respectively having flow rate of 300m³/h in WHRB. The temperature raised during NO_X production was reduced to 280^oC and pressure 5 kg/cm² by this steam. The scale thickness on the inner side of the boiler shell was calculated to estimate Larson Miller Parameter (LMP) which in turn was used to predict the temperature of the shell at the time of failure. The spheroidization and triple point cracks were observed in the microstructures which validated the long term creep failure. The major crack was intergranular and growth was along the embrittled grain boundaries. It was found that the failure occurred in the upper section of WHRB shell corresponded to prolong heating and overheating beyond the design limits.