Abstract: The comparative anatomy of the leaves of 23 species, 4 varieties and 4 forms of the genus Pinus has been investigated with special reference to their transverse sections and epidermal structures. Following a brief description of each species, a key to such is proposed. The stomata of these species can be classified into two types, each represented by P. koraiensis (Haploxylon) and P. tabulaeformis (Diploxylon) respectively. Based on the fact that the epidermal character is intimately correlated with the arrangement of the “connecting cells” in the stomata, it is reasonable to subdivide the genus Pinus into two subgenera. Likewise, the characteristics of the hypodermis, the position and number of the resin ducts, and the nature of endodermis are important criteria for the identification of these two subgenera. The systematic position of P. bungeana Zucc., P. yunnanensis Franch. var. tenuifolia Cheng et Y. W. Law, and P. kwangtungensis Chun has also been discussed.