DEDD is a member of the death-effector domain protein family.  DEDD inhibits the Smad3 mediated transcriptional activity and participates in the regulation of apoptosis.  In this study, how the death-effector domain of DEDD participates in the regulation of Smad3 activity and apoptosis has been further investigated.  Immunoblotting, immunofluorescence and immunoprecipitation had been used to detect the effects of the full length DEDD and its two truncated mutants, N-DEDD and C-DEDD on Smad3 subcellular distribution, phosphorylation, and interaction between Smad4.  The effects of the full length DEDD and its two truncated mutants on cell apoptosis and proliferation had also been explored by flow cytometry and MTT assay.  It showed that DEDD and N-DEDD inhibit TGF-β1 induced Smad3 nuclear translocation and the formation of Smad3-Samd4 complex.  DEDD and its two mutants can induce cell apoptosis and inhibit cell proliferation.  These results suggested that DEDD inhibits the activity of Smad3 through its death-effector domain.  Both the two truncated mutants of DEDD participate in the regulation of apoptosis and cell proliferation.