Transforming growth factor (TGF)‐β signaling has been shown to promote tumor growth and metastasis in advanced cancer. Use of inhibitors of TGF‐β signaling may thus be a novel strategy for treatment of patients with such cancers. In this study, we investigated the effects of a novel TGF‐β type I receptor (TβR‐I) kinase inhibitor, Ki26894, on bone metastasis of a highly bone‐metastatic variant of human breast cancer MDA‐MB‐231 cells, termed MDA‐MB‐231–5a‐D (MDA‐231‐D). Ki26894 blocked TGF‐β signaling in MDA‐231‐D cells, as detected by suppression of phosphorylation of Smad2 and inhibition of TGF‐β‐responsive reporter activity. Moreover, Ki26894 decreased the motility and the invasion of MDA‐231‐D cells induced by TGF‐βin vitro. Ki26894 also suppressed transcription of plasminogen activator inhibitor‐1 (PAI‐1), parathyroid hormone‐related protein (PTHrP), and interleukin‐11 (IL‐11) mRNA of MDA‐231‐D cells, which were stimulated by TGF‐β. X‐ray radiography revealed that systemic Ki26894 treatment initiated 1 day before the inoculation of MDA‐231‐D cells into the left ventricle of BALB/c nu/nu female mice resulted in decreased bone metastasis of breast cancer cells. Moreover, Ki26894 prolonged the survival of mice inoculated with MDA‐231‐D cells compared to vehicle‐treated mice. These findings suggest that TβR‐I kinase inhibitors such as Ki26894 may be useful for blocking the progression of advanced cancers. (Cancer Sci 2007; 98: 127–133)