A two-hectare plot of a deciduous dipterocarp-oak forest of Doi Suthep-Pui National park of northern Thailand protected against fire for 28 years was compared with a similar, but frequently burnt forest nearby with respect to changes in plant diversity. The objective of the study was to find out the effects of forest fire protection on plant diversity. To survey the tree communities, six meter wide transects with a total length of 650m in each site were laid out across the slope of the mountain following a bearing of 600. To survey the ground flora, quadrates of 2×2 m2 area were placed in each site covering 2.3% of the total transect area. The importance value percentage (IP) of trees > 10 cm DBH, species composition and diversity for both tree and ground flora communities were calculated. The species richness of both the ground flora and tree species was higher in the protected areas. The occurrence of evergreen or tropophyllous trees was greater in the protected area than in the burnt area.