We examined the cell death-inducing property of human Fas-associated factor 1 (hFAF1) in the heat shock signaling pathway. By employing co-immunoprecipitation and peptide mass fingerprinting using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, we found that hFAF1 binds to the 70-kDa heat shock protein family (Hsc70/Hsp70). Interaction mapping indicated that the 82-180 sequence of hFAF1 directly binds to the N-terminal region containing sequence 1-120 of Hsc70/Hsp70. This binding is very tight regardless of ATP and heat shock treatment. Hsc70/ Hsp70 and hFAF1 co-localized in the cytosol and nucleus and concentrated to the perinuclear region by heat shock treatment. We examined how hFAF1 regulates Hsp70 function, and found that hFAF1 inhibited the Hsp70 chaperone activity of refolding denatured protein substrates, accelerated heat shock-induced SAPK/ JNK activation, and raised heat shock-induced cell death in a binding dependent manner. These results suggest that hFAF1 prevents cells from recovery after stress by binding to and inhibiting the chaperone activity of Hsp70.