In Cambodia, 70 percent of the population has no access to piped water, despite 245 private operators licensed to provide this public service. Of the remaining 30 percent that does have access to piped water, Cambodia’s mountainous area to the North East has the least coverage and lowest access capability. There is a lack of research that focuses on the factors associated with the piped water sustainability function especially that of private service providers in Cambodia and its mountainous area. To obtain an evaluation of the current performances of piped water supply and its movement towards or away from sustainability, four criteria, adopted from the SWARD guidebook framework, will be employed. Through utilizing this lens, the effectiveness of the piped water supply delivered by private sector in Cambodia’s mountainous area can be analyzed. This paper shows that, due to the poor performance of four determinants; economic, environmental, social and technical dimension, piped water services in Cambodia’s mountainous region are not operating at optimal levels and fall short of sustainability.