The role of added inorganic salt on the thermal degradation of lignin was studied. The result was compared with the thermal analysis of two types of agricultural residues: rice straw with high silica content and bagasse with low silica. Thermal decomposition of such materials is faster than that of wood with a dual mechanism concept similar to wood. In general, the activation energy for rice straw is low; about 19.3 Kcal/mole, while that of bagasse is 33.4 Kcal/mole. This proves that the inorganic material, either inherited or added changed the thermal activities of the lignocellulosic materials.