Traditional building materials based on earth offer the following advantages: easily accessible, inexpensive, and above all, environmentally friendly. This investigation is intended to determine the thermal conductivity of traditional building materials obtained by mixing two kinds of soil (agricultural and soil taken from a type of desert land (Hamada)), separately, with different amounts of straw, according to the expertise of the inhabitants of M'Sila (a province of Algeria). Then, find the best mass proportion (soil/straw), which gives the lowest thermal conductivity of the obtained earthen materials. Four samples were prepared for each soil, in addition to the reference one (without straw). The considered samples were obtained by sun-drying of the handy dough formed by soil, straw, and water. The hot wire method is the technique used to determine the thermal conductivity in longitudinal and transverse directions. All the tested samples display a thermal anisotropy as the thermal conductivity measured in the longitudinal direction is significantly different from that measured in the transverse direction (p < 0.05). The thermal conductivity of the tested samples is strongly affected by the quantity of the straw added (p < 0.05). All the samples tested with and without straw are within the range ([0.1, 2] W/m° C) recommended for construction materials. It was observed that adding a straw mass equivalent to one-twentieth of the mass of both soils (5% by mass) gives to the Adobe formed a minimum thermal conductivity. The thermal conductivity of the studied earthen based materials depends on their compositions.