The number of slope failure cases in Brunei Darussalam has been observed to increase during periods of intense rainfall, such as during the month with peak rainfall. Like other countries around this region, a recommended value of slope angle during earthwork and construction has been established. However, the apparent correlation between landslides and high rainfall intensity suggests that an investigation needs to be carried out to see whether the recommended value is appropriate for the current Brunei's climate condition, especially in relation to a potential reduction in the factor of safety due to rainfall infiltration. This study investigated the effect of the peak monthly rainfall distribution on the stability of different slope angles using the continuous rainfall data from December 2013 to January 2014 as a case example. A commercial software, GeoStudio2012, was used to carry out the numerical analysis of rainfall infiltration and limit equilibrium to evaluate the factor of safety of slopes. The study revealed that the current recommendations may not be appropriate for certain soil condition under the considered critical rainfall distribution. It also revealed that the daily highest intensity of rainfall did not necessarily result in a slope failure, but rather the accumulation of rainfall was a more significant factor to trigger slope instability.