This study examines the effects of doubled CO₂ concentration on the ultrastructure and function of chloroplasts from cycads and, for control from two other herbaceous angiosperms. Under a doubled CO₂ concentration condition, the chloroplast ultrastructure of the two cycads (Cycas multipinnata with a shade-type chloroplast and C. panzhihuaensis with a sun-type chloroplast) changed little: The conformation of the thylakoid membrane system kept well, and almost no starch grains accumulated. In contrast, under the same conditions the chloroplast ultrastructure of soybean and foxtail millet changed considerably, with starch grains accumulating in their chloroplasts and some of thylakoids (especially stroma thylakoid) membranes being destroyed to some degree by the more numerous and larger starch grains that accumulated in the chloroplasts. Interestingly, the changes in the ultrastructure of the chloroplasts from the two cycads was correlated with the 77K fluorescence emission spectra of their chlorophyll; i.e., the F685/F734 (PS II / PS I) ratio within the chloroplasts, which were minimal. The absorption spectrum showed decreases in the red and blue peaks. These changes in the absorption spectrum may be related to changes in the structural arrangement of the thylakoid membranes. Preliminarily, this experimental result shows that the cycads may adapt themselves to environmental changes under doubled CO₂ concentration in the coming centuries. However, more studies on this aspect are necessary.