Abstract: In this paper, an investigation of the law of change of internal stresses during the drying process of timber is presented. According to the principle that within the range of elastic limit the stresses of timber is in direct proportion to its elongation, a measuring method of internal stresses of timber has been devised. With the self-designed measuring device, the changes of the internal stresses of Korean pine (Pinus Koraiensis Sieb et Zucc) and Manchurian ash (Fraxinus mandshurica Rupr.) have been determined, which reveals the peculiarities of the internal stresses of these two kinds of timber during the drying process. It shows that the internal stress varies in three stages in the process of drying: in the first stage it is tensile outside and compressive inside, in the middle stage in the transition of internal stresses and in the last stage tensile inside and compressive outside. The effect of the terminal treatment on the elimination of internal stresses is also discussed in this paper. Changes of the internal stress of larch (Larix olgensis A. Henry) and Manchurian maple (Acer mono Ma xim.) during their terminal treatment have been tested and measured. The result shows that the internal stress of timber can be obviously reduced through terminal treat ments, thus confirming the significance of the terminal treatment.