Abstract: In order to study the growth characteristics of Atlas cedar (Cedrus atlantica Manetti) and to assess the plant diversity an investigation was carried out in a 30 year old man-made stand located in the Caspian mountain forests (north of Iran). In this stand a silvicultural treatment as thinning was operated in five replications of 200 m2 for removing 25% of basal area. Likewise five replications of 200 m2 were considered as control (unthinned). The results of 5 year investigation revealed that diameter growth of C. atlantica was two times greater in thinned stand than in unthinned stand (p<0.05). Height growth and basal area (b.a.) in thinned stand were not greater than those in controlled stand. Vitality quality in both treatments was fairly high. Fruit production was significantly more ample in thinned stand than in unthinned stand. Compared to unthinned stand, in thinned stand index of Simpson Diversity was greater but indices of Menhinic Richness and Peet Evenness were smaller. This study reveals that generally diversity has been increased in the managed stand. Besides that C. atlantica has been adapted to this region and therefore thinning practice can be a good measure for increasing timber production.