An aberrant expression of integrin β1 has been implicated in breast cancer progression. Here, we compared the cell behaviors of wild-type (WT), β1 gene deleted (KO), and β1 gene restored (Res) MDA-MB-231 cells. Surprisingly, the expression of β1 exhibited opposite effects on cell proliferation. These effects were dependent on cell densities, and they showed an up-regulation of cell proliferation when cells were cultured under sparse conditions, and a down-regulation of cell growth under dense conditions. By comparison with WT cells, the phosphorylation levels of ERK in KO cells were consistently suppressed under sparse culture conditions, but consistently up-regulated under dense culture conditions. The phosphorylation levels of EGFR were increased in the KO cells. By contrast, the phosphorylation levels of AKT were decreased in the KO cells. The abilities for both colony and tumor formation were significantly suppressed in the KO cells, suggesting that β1 plays an important role in cell survival signaling for tumorigenesis. These aberrant phenotypes in the KO cells were rescued in the Res cells. Taken together, these results clearly showed the distinct roles of β1 in cancer cells: the inhibition of cell growth and the promotion of cell survival, which may shed light on cancer therapies.