Quantification of rock slope stability analysis in Malaysia has generally been limited to the traditional methods of Rock Mass Rating (RMR) or Slope Mass Rating (SMR). While these methods represent a means to convey message from geologists to engineers, they can be confusing especially in designing slope stabilization efforts. The limit equilibrium method which is widely used worldwide has only seen limited application for rock slope stability analysis in Malaysia. This paper discusses the limit equilibrium method using Factor of Safety (FOS) from deterministic calculations to probabilistic analysis. Four rock slopes in Klang Valley (L1, L2, L3 and L4) with potential and failed planar failures have been selected for FOS calculation. In this study, Barton�s equation has been selected in favour of the Mohr-Coulomb method due to the availability of Joint Roughness Coefficient (JRC) data. A deterministic calculation in dry conditions shows that FOS values range from 1.08 to 2.34 which indicates stable conditions. However, introduction of water into the discontinuities (50 filling) reduces the FOS value from 0.80 to 1.89 while 100 filling of water inside discontinuities further reduces the FOS value to the range from 0.00 to 0.55. The results indicate the importance of water in influencing the FOS calculations. Probabilistic analysis uses normal distribution of parameters measured at the field except for water content in which an exponential distribution is assumed. The probabilistic FOS analysis shows probability of failure ranging from 4.83 to 44.80 whereby the slope with highest likelihood of failure is at L1. Estimated block weight shows median mass value between 0.07 tonne/m to 1.30 tonne/m. The establishment of FOS and probability of failure values will help the engineers better to design adequate stabilization structures if needed. © 2019 Geological Society of Malaysia. All rights reserved.