This paper presents basic engineering properties of a gneissic residual soil from a natural slope in which a greater landslide occurred. Emphasis is given to the design strength parameters obtained from laboratory tests. A stability analysis is done taking into consideration unsaturated condition. The landslide causal factors are also presented and discussed. The study results have shown that considering a low value of initial soil matric suction in the field the strength parameters mobilized in the first time failure are characterized by the laboratory results (peak or deformation near to the critical state condition).