A study of effect of weathering grade on mechanical properties of rocks have been conducted on selected rock samples from Crocker Formation, Trusmadi Formation and peridotite on weathering grade II to V. The objective is to study on how weathering grade does able to alter the microfabric of rocks which affect to its mechanical properties. The increment of weathering grade does increase the percentage of microfractures, porosity and clay minerals formation which became the factor of rock weakness through the reduction of density, hardness and strength of the rock. By understanding this, it is easier to predict and estimate the strength of rock slope and slope failure occurrences. As conclusion, sandstones from Trusmadi Formation is the strongest compared to sandstones from Crocker Formation and peridotite with 187.06 – 2.48 MPa, 83.15 – 0.43 MPa and 125.27 – 20.92 MPa throughout weathering grade II to V, respectively