Abstract: In response to the design requirements for high-temperature load-bearing components in marine and shipboard gas turbines to operate efficiently and reliably within confined engine room spaces, the size-dependent mechanical behavior of thin-walled nickel-based superalloy components has drawn increasing attention. In this study, a casing casting was fabricated via a counter-gravity casting method. Tensile specimens with thicknesses of 1 mm, 2 mm, and 3 mm were machined from the flange region of the casting, which originally had a wall thickness of 10 mm. The influence of specimen thickness on room-temperature tensile properties, fracture behavior, and underlying deformation mechanisms was systematically investigated. The results showed that as the specimen thickness decreasing, all the tensile strength, yield strength and fracture elongation might decline markedly, indicating a pronounced thin-wall effect. Fractographic analysis revealed a transition in fracture mode from typical ductile failure dominated by dimples in thicker specimens to quasi-brittle fracture in thinner ones. Electron backscatter diffraction results indicated that thin-walled specimens exhibited higher kernel average misorientation values during tensile deformation, reflecting greater local strain heterogeneity and more severe dislocation accumulation. This is primarily attributed to the reduced geometric constraint on grains and the significantly increased fraction of surface grains resulting from wall-thinning, which collectively impair the alloy’s capacity for coordinated plastic deformation. Interrupted tensile tests further confirmed that microvoids and microcracks might initiate at an early stage of plastic strain in thin specimens and propagate rapidly. These findings elucidate the intrinsic mechanisms of the thin-wall effect in K439B alloy from the perspectives of microscale deformation and damage evolution, providing experimental evidence and theoretical insight for the structural design and service performance assessment of thin-walled nickel-based superalloy components.