Abstract: Effect of CO₂ enrichment on the accumulation of nonstructural carbohydrate (NSC) in tomato plants was studied in a greenhouse. The atmospheric CO₂ concentration was maintained at 400±25 μmol/mol (CK), 600±25 μmol/mol (T1), 800±25 μmol/mol (T2), or 1, 000±25 μmol/mol (T3) during the entire growth period of Xinghai No.12 tomato plants. Contents of glucose, fructose, sucrose and starch in the sources and sinks of individual plants were determined. The results indicated that CO₂ enrichment increased the NSC in various organs of a plant. As the plants grew and developed, the glucose content in the leaves peaked when the fruits were ripening. The content increased 94% over CK under T3 when CO₂ at the highest level. At T3, the sucrose content remained highest among all treatments throughout the entire growth stages. The starch in leaves was relatively low under T2. High CO₂ content in the air increased NSC in the vascular organs of the plants, especially the stems, which had the highest content in the first 3 stages of fruit development under T3. In the sepals, the enriched CO₂ did not show any significant effect on NSC. However, it hastened the breakdown of sucrose and starch increasing 46% on glucose and 42% on fructose accumulations in the fruits under T2. It appeared that CO₂ enrichment could promote the NSC accumulation in various parts of a tomato plant and transport the photosynthates from the source to the sink improving the eating quality of the fruits. Over all, the T2 treatment seemed to be optimal for the purpose.