Changbai mountain is a very important natural reserve in China as well as in the world, because it possesses a high bio-diversity, and is a typical ecosystem representing boreal vegetation. Few studies on the dynamic-equilibrium of forest in relation with natural disturbance have been reported. In this study, the change of forest vegetation in the reserve was detected by using Landsat TM images. Image differencing between 1984 and 1997 was adopted to derive new images that indicate cover type change. The natural forest in the reserve was in a status of relative equilibrium. The number of pixels with decrease in radiance was nearly the same as those with increase in radiance. It is demonstrated that the so-called climax is not a pure stand which is generally assumed to be exclusively dominated by climax species, but a complex of mosaic structure that consisted of patches in different stages of succession. This climax structure was maintained by natural disturbance like fall. The pioneer patches are permanent units in the forest community. Gaps are frequently created, and thus the pioneer patches are kept constantly. As a whole, TM imagery is effective for detecting vegetation changes, but tiny gaps with several pixels are difficult to discriminate from noise. The change inside the natural reserve was minor, while the vegetation outside the natural reserve presented an upgrading status, showing the recovery after timber cutting.