Abstract: The present study was conducted in natural Himalayan forests of Parsuram Kund area in Lohit district of Arunachal Pradesh to understand the effect of altitudinal variation on structure and composition of the vegetation. Three altitudinal zones viz., upper zone (U) = 550-850 m asl, middle zone (M) = 500-700 m asl and lower zone (L) = 350-450 m asl were selected for the study. Tree Species richness (SR) was recorded to be highest (26) on the middle altitude followed by lower (21) and upper altitude (13). Species diversity (richness) and dominance (Simpson Concentration of dominance index) were found to be inversely related to each other. Shrub and herb species richness were also recorded to be higher on middle altitude followed by lower and upper altitudes. [Nature and Science 2010;8(2):27-35]. (ISSN: 1545-0740).