Abstract: Chrysolaminaran is a water soluble polysaccharide that accumulates in cytosolic vacuoles and UDP-glucose (UDP-Glc) serves as the substrate for chrysolaminaran synthesis. In higher plants, UDP-glucose pyrophosphorylase (UGPase) is a key enzyme in carbohydrate metabolism, catalyzing the reversible production of UDP-Glc and pyrophosphate (PPi) from glucose-1-phosphate (Glc 1-P) and UTP. Our previous findings suggested that UGPase is a rate-limiting enzyme and may play an important role in chrysolaminarin biosynthesis in Phaeodactylum tricornutum. Therefore, theoretically speaking, increasing the UGP gene level is helpful to improve the accumulation of chrysolaminaran. UGPase activity was affected by transcriptional regulation with some environmental factors such as phosphate deficiency stress, cold, light exposure, sucrose feeding, salinity and cadmium excess. However, little is known about the expression patterns of UGP gene by environmental factors in P. tricornutum. Cadmium has been recognized as one of the most toxic aquatic contaminants. Evidence that microalgal cadmium tolerance achieved by metal-chelating peptides was obtained with Cadmium toxicity. It has been widely thought that Zinc has many known biological functions. Under conditions of Zn limitation, Cd enhances the growth rate of some marine phytoplankton in culture, and it thus seems that Cd may replace Zn in some essential biochemical function. To explore the effects of cadmium on the growth and expression of UDP-glucose pyrophosphorylase (UGPase) gene in Phaeodactylum tricornutum, different concentrations of Cd²⁺ were used to cultivate P. tricornutum and the growth, chlorophyll fluorescence parameters, UGP gene transcription level, UGPase activity and chrysolaminarin content were determined. The results showed that low concentration of Cd²⁺ (0.1 μmol/L) promoted the growth of P. tricornutum, and it increased the UGP gene transcriptional level, UGPase activity and the chrysolaminarin content by 100.65%, 11.99% and 9.77%, respectively, compared with those of the control group. High concentration of Cd²⁺ (2 and 5 μmol/L) significantly decreased all measurements. The UGP transcriptional level, UGPase activity and the chrysolaminarin content were decreased by 50.31% and 60.47%, 56.27% and 66.72%, 42.41% and 47.30%, respectively, compared with the control group. To our knowledge, this was the first time to investigate the effects of cadmium on UGP gene expression in P. tricornutum. The results revealed that low concentration of cadmium can enhance the growth of P. tricornutum, induce the expression of UGP gene and promote the synthesis of chrysolaminarin, which provide a reference for studying the expression of UGPase gene regulated by cadmium.