In order to understand the toxic effect of terbium (Tb(III)) on the plant photosynthesis, we investigated the photosynthesis, the ultrastructural structure of chloroplast, the subcellular distribution of horseradish peroxidase (HRP), the activity of HRP, and the content of malondialdehyde in horseradish by a portable gas exchange system, transmission electron microscopy and the other biochemical technologies. The results indicated that after horseradish treated with 5 mg L(-1) Tb(III), the subcellular distribution of HRP was not obviously changed. However, the activities of guaiacol and ascorbate HRP were decreased comparing with that of horseradish treated without Tb(III). It could cause the peroxidation of membrane lipid, the damage of chloroplast ultrastructure and the decrease in the photosynthesis and the content of chlorophyll. Moreover, after horseradish treated with 60 mg L(-1) of Tb(III), the distribution of HRP on the plasma membrane and tonoplast was decreased, while the distribution of HRP on the cell wall was increased comparing with that of horseradish treated without Tb(III). The change in the subcellular distribution of HRP could induce the excess accumulation of the reactive oxygen, leading to the damage of the chloroplast ultrastructure and then the decrease in the photosynthesis. Furthermore, the effects of Tb(III) on the indexes mentioned above were increased with prolonging the treating time of Tb(III). These results demonstrated that the function of HRP in horseradish treated with Tb(III) was decreased, leading to the damage of chloroplast ultrastructure and then the inhibition of photosynthesis. It was a possible toxic effect of Tb(III) on the plant photosynthesis.