A failure had occurred in a petrochemical plant in a weld joint of a de-superheater only six months after being in service. System was commissioned after making necessary repairs using same metallurgy since all the needed resources were long delivery-scheduled items. Leaking of steam through the weld joint revealed the failure location. Analysis of the failed samples by metallurgical techniques, none destructive testing (NDT), scanning electron microscopy (SEM), and Energy Dispersive X-ray (EDX) techniques indicated that the failure was caused by improper design of the injection of the BFW in the de-superheater that producing inner cyclic thermal differential loads leading to thermal fatigue. This paper will discuss the mechanism of the prevailing thermal fatigue failure was explored and explained.