Protein phosphatase 1α (PP1α) regulates phosphorylation of BRCA1, which contains a PP1-binding motif ⁸⁹⁸KVTF⁹⁰¹. Mutation of this motif greatly reduces the interaction between BRCA1 and PP1α. Here we show that mutation of the PP1-binding motif abolishes the ability of BRCA1 to enhance survival of Brca1-deficient mouse mammary tumor cells after DNA damage. The Rad51 focus formation and comet assays revealed that the DNA repair function of BRCA1 was impaired when the PP1-binding motif was mutated. Analysis of subnuclear localization of GFP-tagged BRCA1 demonstrated that mutation of the PP1-binding motif affected BRCA1 redistribution in response to DNA damage. BRCA1 is required for the formation of Rad51 subnuclear foci after DNA damage. Mutation of the PP1-binding motif in BRCA1 also affected recruitment of Rad51 to sites of DNA damage. Consistent with these findings, knockdown of PP1α in BRCA1-proficient cells by small interfering RNA also significantly reduced Rad51 focus formation induced by DNA damage. Further analysis indicated that mutation of the PP1-binding motif compromised BRCA1 activities in homologous recombination. Altogether, our data implicate that interaction with PP1α is important for BRCA1 function in DNA repair.