The watershed forests at Cua Dat reservior have important ecological functions in water conservation and soil erosion prevention. However, the forests in the area is mainly the degredaded secondary forest, and have not been evaluated in terms of species diversity and structure as the basis for determining an appropriate forest structure functions of a watershed protection vegetation. This research results from 12 plots on species diversity and structural characteristics of the forest types showed that the forests of Cua Dat reservior are rather diversify in terms of species richness with 23-48 trees species per 2000 m2 plots. The species diversity index is relatively high ( 2.649 to 3.431), and abundance level varies from 2.277 to 3.863), but there is a large degree of variation between the large states . In rich and medium forest, there have appeared a number of communities with dominant tree species belonging to the group of shade tolerant valuable timber trees. Sapling density under conopies of stocked forest ypes are generally rather high ( 3000 saplings / ha) with the high percentage of good quality ones (over 55 percent) . The leaf area index (Cai ) reaches relatively high values, from 150 to 300 percent, but the ground cover is at moderate level (approximately 40 percent) and varies greatly among the forest types. In the last 5 years, these cover indicators is increasing by time. The inceament of Cai (Zcai) of the forest vegetation in the study area can be estimated by the following equation with high level of correlation coefficient (0.877): Z ( Cai ) = 6.237 - 0.035 x Cai + 6.537 * Cai K5 * Cae.