This study was carried out to know varietal differences in net CO_2 assimilation rate and leaf area increasing rate and their changes in breeding process of main cu1tivars of sweet potato improved in Japan. The cultivars used in this study were 7 native cultivars, 14 cultivars bred before 1955 and 10 cultivars bred after 1955 (Table 1). They were grown in pot culture for measurement CO_2 assimilation and transpiration, and in field for survey of leaf area increasing rate, respectively. Rates of CO_2 assimilation and transpiration were simultaneously measured by a climatic-controlled assimilation chamber method in September, 1982. Leaf area increasing rate was calculated from multiplication of leaf emergence rate and mean area per leaf for 12 days surveyed in September, 1982. Results obtained were as follows, 1. There were large varietal differences in CO_2 assimilation rate. However, most of improved cultivars were higher in CO_2 assimilation rate as compared with native cultivars. 2. There also were varietal difference in transpiration rate. Transpiration rate of improved cultivars was higher than that of native cultivars. 3. There was a high positive correlation between CO_2 assimilation rate and transpiration rate. Most of improved cultivars were higher in both rates of CO_2 assimilation and transpiration as compared with native cultivars. 4. There were large differences in dark respiration rate. Cultivars bred after 1955 were larger than that of cu1tivars bred before 1955. 5. Leaf conductance had high positive correlations with CO_2 assimilation rate and transpiration rate, respectively. Leaf conductance values in improved cu1tivars were higher than that of native cultivars. 6. CO_2 assimilation rate had significant positive multiple-correlation with leaf conductance and mesophyⅡ conductance. However, CO_2 assimilation rate correlated more closely with mesophyⅡ conductance than leaf conductance MesophyⅡ conductance was larger in improved cultivars as compared with native cultivars. 7. Leaf emergence rate (A), mean area per leaf (B) and leaf area increasing rate (A×B) in cultivars bred after 1955 were larger as compared with ones bred before 1955. Furthermore, specific leaf area (SLA) in improved cultivars was larger than that of native cultivars. 8. It was made clear from mentioned above that most of cultivars bred after 1955 had high abilities of CO_2 assimilation and leaf area increasing. At the same tame, it suggests that crossing and selection in breeding process of sweet potato in Japan were performed for increase of abilities in CO_2 assimilation and leaf area production with balance of them.