Belled piles are increasingly being employed for transmission tower foundations in the mountainous regions of Western China, but their uplift behavior is not fully understood. This paper presents experimental investigations of the pull-out behavior of belled piles in horizontal and inclined ground (overlying soil and underlying soft rock). 3D non-linear finite element analyses related to the model tests were also conducted using ABAQUS software, and the influence of slope effect on the ultimate uplift capacity of rock-socketed belled piles was evaluated. The results demonstrated that the inclined ground led to deterioration in its uplift performance, thus increasing both the displacement of the pile cap and ground surface. When the slope angle changes between 0° and 30°, the weakening effect of the slope on the ultimate uplift capacity increases linearly. When the slope angle is increased to 45°, the weakening effect of the slope on the ultimate uplift capacity is greatly enhanced, and the relationship between the ultimate uplift capacity of belled piles in the inclined ground and the slope angle is proposed. Moreover, the failure mode of the horizontal bedrock is an inverted cone with a failure angle between 0.8 and 1.0φ_r (φ_r is the internal friction angle of bedrock). In contrast, the bedrock failure of inclined ground is bulb-shaped, and it mainly occurs within 4d (d represents pile diameter) of the downslope side. It was also discovered that the downslope side of the belled pile in the sloped ground generated cracks that directed to the pile axis if the pile top load reached 82.3% of the ultimate uplift capacity. These findings are valuable for practicing engineers in the rational design of belled piles, taking account of the weakening effect of inclined ground.