Inducible cyclooxgenase-2 (COX-2) is commonly overexpressed in breast tumors and is a target for cancer therapy. Here, we studied the effect of Celecoxib, a COX-2 inhibitor on two molecular breast cancer subtypes-MDA-MB-231 and SK-BR-3. Firstly, MDA-MB-231 and SK-BR-3 cells were treated with various concentration of Celecoxib for 24 and 48 h. Celecoxib-inhibition of NF-kappa B (p52 and p65) transcriptional activity and effect of Caspase 3 pathway were examined by western blotting. COX-2 mRNA was assessed by RT-PCR. Cell viability was evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assay. Both cell cycle profiles and apoptosis were analyzed using flow cytometry. We found that Celecoxib inhibited the proliferation of the MDA-MB-231 cell line in a dose time dependent manner versus SK-BR-3 in a dose dependent manner only (p < 0.05). Celecoxib induced apoptosis of the MDA-MB-231 and SK-BR-3 cell lines in a dose-time dependent manner (p < 0.05) with more mean apoptotic cells in MDA-MB-231 than SK-BR-3. Significant cell-cycle arrest at the G1 phase in the MDA-MB-231 versus G2 phase in SK-BR-3 cell lines. NF-kappa B (p52 and p65) and COX-2 expressions were downregulated in a dose dependent manner, while Caspase 3 expression was upregulated in both cell lines. In this present study, our data indicated Celecoxib might affect each breast cancer subtype independently. Therefore, when using Celecoxib in treatment of breast cancer, it is imperative to consider the subtype of breast cancer on a molecular level. (C) 2017 Published by Elsevier Inc.