The Sultan Ibrahim Reservoir located at Gunung Pulai catchment area was previously managed by the Singapore’s Public Utilities Board before the Board handed over the reservoir to Johor State Government under current management of the Syarikat Air Johor. At the present time, the Syarikat Air Johor is operating a Gunung Pulai Water Treatment Plant consisted of two main plants, i.e. Water Sedimentation Plant and Water Filtration Plant for treating raw water from the reservoir before supplying to the consumers. However, the integrity of water treatment plant’s structure should be checked due to several cracks were observed, and moreover the structure was built over more than 90 years. The formation of the cracks in the Gunung Pulai Water Treatment Plant may be induced by movement of the sloping area to the south-east of Water Sedimentation Plant. The calculated Factor of Safety (FOS) via sensitivity analysis for cross sections of original slope indicates any decrease in friction angle and/or cohesion strength or increase in horizontal seismic load will further cause instability on slope. Also, the results of Load-Deformation analysis on cross sections of original slope show significant vertical displacement and horizontal displacement on the bottom and both sides of sedimentation tanks, respectively. The results indicate applied structural and water loads significantly affect deformation at both vertical and horizontal directions which could have contributed to FOS < 1 in slope stability analysis.