The ultrastructural aspects of Cyperus iria leaves showing the C4 syndrome and the typical C3 species, Carex siderosticta, in the Cyperaceae family were examined. C. iria exhibited the chlorocyperoid type, showing an unusual Kranz structure with vascular bundles completely surrounded by two bundle sheaths. The cellular components of the inner Kranz bundle sheath cells were similar to those found in the NADP-ME C4 subtype, having centrifugally arranged chloroplasts with greatly reduced grana and numerous starch grains. Their chloroplasts contained convoluted thylakoids and a weakly-developed peripheral reticulum, although it was extensive mostly in mesophyll cell chloroplasts. The outer mestome bundle sheath layer was sclerenchymatous and generally devoid of organelles, but had unevenly thickened walls. Suberized lamellae were present on its cell walls, and they became olylamellate when traversed by plasmodesmata. Mesophyll cell chloroplasts showed well-stacked grana with small starch grains. In C. sideroxticta, vascular bundles were surrounded by the inner mestome sheath and the outer parenchymatous bundle sheath with intercellular spaces. The mestome sheath cells degraded in their early development and remained in a collapsed state, although the suberized lamellae retained polylamellate features. Plastids with a crystalline structure, sometimes membrane-bounded, were found in the epidermal cells. The close interveinal distance was 35-50${mu}{ extrm}{m}$ in C. iria, whereas it was157-218 ${mu}{ extrm}{m}$ in C. siderosticta. These ultrastructural characteristics were discussed in relation to their photosynthetic functions.