In this work, we studied the slow pyrolysis process of a Moroccan wood species (Eucalyptus) which is frequently used in the traditional production of wood charcoal, The non-isothermal pyrolysis was investigated by thermogravimetric analysis under inert gas (nitrogen) from ambient to 600 degrees C. We also studied the pyrolysis under the same conditions of xylan, cellulose, lignin and their mixture. The results show that the thermal stability increases from lignin to xylan and cellulose. The charcoal yield of lignin is greater those of xylan and cellulose. The eucalyptus decomposition appears to be a parallel association of these three major constituents. The rates reached by eucalyptus during its pyrolysis at 265 and 325 degrees C were attributed to the decomposition of hemicellulose and cellulose, respectively. In the range 225-370 degrees C, the degradation of lignin is hidden by the hemicellulose and cellulose degradations. The rate af mass loss of eucalyptus is slower than that of xylan and cellulose, but greater than that of lignin.