The distinct roles of estrogen receptors (ERs) related with androgen receptors (ARs) have been proposed in prostate cancer, while the involvement of transforming growth factor-β (TGF-β) has been reported in the progression of prostate cancer. In this study, we examined whether the TGF-β signaling pathway is associated with ER signaling in LNCaP prostate cancer cells, which express ERα, ERβ and ARs. We determined whether the exposure to phthalates may induce prostate cancer progression by affecting molecular crosstalk between ER and TGF-β signaling pathways. Cell viability was measured in LNCaP cells by MTT assay following treatment with di-n-buthyl phthalate (DBP). RT-PCR and immunoblot assay were performed to examine the expression levels of cell cycle-related genes and the TGF-β signaling cascade. A mouse xenograft model of prostate cancer was generated, and immunohistochemical and BrdU assay were carried out to determine the effect of DBP in this mouse model. DBP, a type of phthalate, was shown to promote LNCaP cell proliferation by upregulating the gene expression of c-myc and cyclin D1 and by downregulating the expression of p21. DBP significantly reduced the protein expression of p-smad similarly to E2. These regulations caused by DBP were reversed by ICI 182,780, an ER antagonist, indicating that DBP may affect crosstalk between TGF-β and ER signals. In an in vivo mouse model, tumor volume of mice exposed to DBP was increased. Number of cells in S phase of cell cycle was increased by DBP, while expression of p21 protein was reduced in the tissues of DBP-treated mice. These results indicate that DBP may induce the growth of LNCaP prostate cancer by acting on the crosstalk between TGF-β and ER signaling pathways.