Arsenic, a high toxic heavy metal, is one of the most serious contaminants in soil and potential threat to crop growth and human health. Carbon metabolism plays an important role in growing period and formation both of yield and quality in flue-cured tobacco. It is necessary to study arsenic toxicity to carbon metabolism in flue-cured tobacco. Therefore, a systematic experiment was conducted on the farm of Henan Agricultural University, Zhengzhou, China. Using Yunyan 85 planted in 400 mm×340 mm plastic pots, treatments were layed out with 5 arsenic levels of 0 (CK), 5, 20, 40, and 60 mg•kg^-1 [added as sodium arsenate （Na3AsO4•12H₂O）] to determine the effects of arsenic toxicity on metabolism of carbon. The results showed that arsenic toxicity significantly affected carbon metabolism during the whole growing period. Chlorophyll content, photosynthetic rate and sucrose synthetase (SS, synthetic direction) activity of the leaves during the growing period, as well as activity of sucrose phosphate synthetase (SPS) after budding stage were all decreased. But activity of SS (decomposition direction) and content of soluble sugar during the growing period increased. The results suggested that carbon assimilation and sucrose synthesis in flue-cured tobacco be inhibited, and sucrose decomposition be accelerated as a result of arsenic toxicity, which was not favorable for carbon transformation. It was also indicated that activity of soluble starch synthetase (SSS) both at rosette stage and harvest stage, and accumulation of starch at harvest were inhibited, SSS activity at budding stage, activity of adenosine diphosphoglucose pyrophosphorylase (ADPG-PPase) during the growing period, and starch content both at rosette stage and budding stage were all increased by arsenic toxicity. Therefore, arsenic toxicity caused the disorder of carbon metabolism and decrease in starch content during the later part of the growing period.