The fatigue behavior and structural integrity of a 3-inch butterfly valve shaft were analyzed using finite element analysis (FEA), Goodman mean stress theory, and experimental validation to assess failure mechanisms under cyclic loading. The shaft, made of SS 316L steel with a tensile yield strength of 205 MPa and ultimate tensile strength of 515 MPa, exhibited stress concentrations in transition zones, with maximum von Mises stress reaching 506.15 MPa and total deformation of 2.6777 mm. Fatigue life analysis identified regions experiencing as few as 50 cycles before failure, emphasizing the need for geometric optimization. Microstructural analysis using Scanning Electron Microscopy (SEM) confirmed fatigue crack initiation at machining marks and propagation along grain boundaries, aligning with FEA predictions. The Goodman mean stress theory provided an accurate fatigue life assessment, incorporating mean and alternating stress effects to highlight critical failure regions. To enhance fatigue resistance, future work proposes replacing SS 316L with XM19 steel and validating its performance under simulated operational conditions and cyclic testing. These insights are vital for improving butterfly valve durability and ensuring long-term operational reliability in industrial applications.