Chemoresistance and tumor relapse remain major clinical problems. Evidence indicates that COX2/ PGE2/EP axis has a critical role in tumorogenesis and chemoresistance. This study assessed the relation of the COX-2 gene expression with chemoresistance in colon cancer (CC) patients. Also, it explored the effect of chemotherapy on COX-2 expression. The study included 24 patients with CC without chemotherapeutic treatment and 24 chemoresistant CC patients. Tumor and adjacent non-neoplastic colon tissue samples were collected and COX-2 mRNA expression was measured. Also, COX-2 and its related genes; TROP2 and DUSP4 expression were analysed in 5 flurouracil and Oxalliplatin treated Caco-2 and SW-620 cells. The results indicated significant upregulation of COX-2 expression in tissues of chemoresistant CC patients when compared with that in CC tissues without chemotherapy (p < 0.001). There was a relation between COX-2 expression with lymph nodes, metastases and staging in both groups. Concerning in-vitro experiments, there was a dose dependent significant increase of COX-2, TROP2 and DUSP4 mRNA and protein expression levels in 5flurouracil and Oxalliplatin treated cells. These findings demonstrated that overexpression of COX-2 in the chemoresistant CC patients. Both 5 flurouracil and Oxalliplatin induced COX-2 overexpression and in turn COX-2 upregulation may decrease the response of cancer to chemotherapy.