The effectiveness of protected areas (PAs) has often been measured by comparing deforestation or land cover loss before and after PA establishment or within and outside the PA. Such evaluations ignore forest degradation or the decline in quality of the ecological values of the forest. Landsat image processing and geographical information systems were used to study forest cover changes in and around Popa Mountain Park (central Myanmar). Deforestation inside the Park was lower than outside the Park, but forest degradation, namely the net area that changed from dense forest to open forest, within the Park had grown at rates exceeding those outside the Park. The effectiveness of the PA was overestimated by adopting deforestation as the only indicator of conservation effectiveness. Forest degradation studies can be used to evaluate the effectiveness of PAs.