Abstract: By using the vegetation quardat method and calculating the important values,the dominant families,species and their size classes of arbor layer in the forest communities in Qingzhangshan of Nan-xiong City,Nanshan of Shixing County,Shaxi of Qujiang County,Qingyunshan of Wengyuan County,Yunjishan of Xinfeng County,and Daguishan of Heyuan City,are compared in this paper.The results show that there are 13 common families which are dominant in the forests of 6 research sites,such as Fagaceae,Lauraceae,Theaceae,Hamamelidaceae,Aquifoliaceae,Eriaceae,etc.These families are coincidence with their locations of subtropical region.There are 9 common dominant species in the forests of 6 research sites and their dominance are different from each other.The biggest similarity coefficient of communities is 75.88% between Shaxi and Nanshan,while the lowest is 45.64% between Qingyunshan and Daguishan.All of the similarity coefficients between Nanshan and other sites are more than 60%.Among the size classes of the common dominant species,the individuals amount and their distribution of the population of Castanopsis eyrei,Diospyros morrisiana,and Choerospondias axillaries are related to the differences of the latitude of the research sites.The population of Castanopsis fissa in Yunjishan and Shaxi present an increasing trend,while the population in Nanshan and Daguishan present a decreasing trend.The population of Schima superba in Qingyunshan presents an increasing trend,but the population in other sites are difficult to regeneration.The sizes of Cinnamomum porrectum population in each site are small and reflect the distribution of the species is scatter type,and the size class is not integrity.The population of Liquidambar formosana in each site are lack of seedlings in stage I showing a not complete size class structure.Basically, the population of Cunninghamia lanceolata and Pinus massoniana present a decreasing trend in each site.The present results suggest that in general the non-clement typical vegetations in 6 research sites of northern Guangdong are decline with time,while the evergreen broadleaf forest develops smoothly.