This study focuses on the influence of weathered rock’s microstructure and its effect to the stability of slope at the Trusmadi Formation. Trusmadi Formation (Palaeocene to Eocene age) had experience a slightly metamorphism process and makes it highly susceptible to instability. Regional metamorphism that happened during tertiary has given rise to the formation of argillaceous rock. This rock then suffering a weathering process that change them to soil. Four samples were collected based on fail slope and stable slope. These samples were analysed using polarizing microscopic and scanning electron microscopic (SEM). Microstructural features identified that all slopes had the same platy minerals and mineral grain size and shape except for the cracks and porosity percentage. Percentage of cracks and porosity in stable slope are 35% and 40% in the average while 55% and 65% in fail slope. For SEM study, the percentage of porosity is 37% for stable slope and 55% for fail slope. The presence of 1:1 clay that is kaolinite gives only a small impact to slope stability.