Together with the excavated width, the depth of trench play a vital role in stability as well as economics effectiveness of a trench constructed by open trench method. It is widely recognized that a steeper angle of an open trench, a smaller amount the excavated soil required; however, the trench is unstable. In this case, an external supporting structure system is obliged to prevent the trench from falling into the excavation zone. In addition, an open trench is normally located above the water table, hence under unsaturated condition. One of the most important characteristics of unsaturated soil is negative pore water pressure which brings about the matric suction, higher negative pore water pressure, higher matric suction; consequently, higher shear strength of soil. On the other hand, there is a few theories and research works have been reported on the method of determining a suitable depth of a trench under unsaturated condition. Previous works tend to assume that the distribution of matric suction is either constant or linear with depth; as a result of this, the designed results are often overestimated compared to practical results. In this paper, the effect of distribution of matric suction was taken into account to propose an equation to estimate the depth of a trench that can be applied without supporting structure. Finally, an example of numerical calculation of a depth of the open trench was described.