Peroxiredoxin I (Prx-I), a key member of the peroxiredoxin family, reduces peroxides and equivalents through the thioredoxin system. Our previous work has shown that expression of Prx-I in mammalian cells increases following ionizing radiation (IR), and suppression of its expression enhances radiation-induced cell death in vitro, suggesting that inhibition of Prx-I might be an important pretreatment for cancer radiotherapy. To test this hypothesis in vivo, we suppressed the expression of Prx-I in the human intestinal cancer cell line SW480 by adenovirus-mediated transfer of siRNA. Our results showed that expression of Prx-I in SW480 cells was dramatically reduced by recombinant Ad-SiPrxI, which resulted in decreased cell growth and increased cell death by IR. Significantly more cell apoptosis was detected by flow cytometry analysis when Prx-I expression was knocked down. To evaluate the effect of recombinant Ad-SiPrxI in vivo, xenografts were pretreated with adenovirus before IR. Tumor growth in mice was inhibited when the xenografts were pretreated with Ad-SiPrxI before IR. Our results suggest that pretreatment with recombinant adenovirus to inhibit Prx-I expression can enhance the radiosensitivity of cancer cells, and thus might be a potential application in clinical therapy.