We studied phylogenetic diversity of tree species richness along an altitudinal gradient from 200 m to 1048 m of Mt. Bokor, a table shaped mountain, locating in the southwest Cambodia. We used two methods to record species richness; recording trees above 4 m in 20 plots of 100 m x 5 m placed from 266 m to 1048 m, and collecting specimens from sea level to 1048 m. We estimated the evolution time of tree species by using rbcL and matK sequences and calculated the phylogenetic α diversity. For both datasets, we applied rarefaction, extrapolation and Chao1 estimator to standardize the sampling efforts. As a result, phylogenetic diversity of tree species observed in 20 plots had no significant correlation with altitude. Phylogenetic diversity estimated by rarefaction, extrapolation or Chao1 from both data sets also showed no significant correlations with altitude. Unlike many previous studies, phylogenetic diversity of tree species was nearly constant along the altitudinal gradient of Mt. Bokor where temperature and precipitation are expected to vary. We suggest that the table shaped landscape may be a determinant of this constant phylogenetic diversity of tree species.