Abstract: The study using plot method to determine floristic composition and forest structure of limestone forests at three different altitudes in Bau have been carried out. A total of 1682 trees encompassing an area of 0.75 ha were enumerated. They belong to 129 species in 41 families. Euphorbiaceae, Moraceae, Ebenacaea, Rubiaceae and Violaceae were important families in term of density. Shannon Diversity Index (H’), species richness (R) and species evenness (E) decreases with increasing altitude. The floristic similarity of family composition is higher compare to species composition between altitudes. Moraceae contributed the highest basal area and above ground biomass contribution in all plots at all altitudes; for species Ficus aurata is dominant at 15 m (P1) and 30 m (P3), whilst Mallotus oblongifolius is dominant at 50 m (P2). Analysis on the importance values (Iv) of all tree species enumerated at all altitudes quantified the dominant and co-dominant species with the highest and second highest Iv. The dominant and co-dominant species is Arenga borneensis and Ficus aurata 15 m; Mallotus oblogifolius and Artocarpus rigidus at 30 m and Ficus aurata Saraca hullettii in 50. Between 68-77% of all species enumerated have Iv less than 5% at all altitudes.