Cyclooxygenase-2 (COX-2) is commonly overexpressed in cancer which catalyzes arachidonic acid peroxidation to produce cancer promoters. Thus, COX-2 inhibition is extensively studied for cancer treatment. However, COX-2 inhibitors suffer from critical side effects and never achieve desirable therapeutic effect in cancer patients. Recent study from our lab showed that siRNA knockdown of delta-5-desaturase (D5D), a key enzyme that converts dihomo-γ-linolenic acid (DGLA) to arachidonic acid, promoted COX-2 catalyzed DGLA peroxidation and formation of an anti-cancer byproduct 8-hydroxyoctanoic acid (8-HOA), which served as an HDAC inhibitor and suppressed growth and migration of colon and pancreatic cancer cells. In the present study, we extend our novel research to mice xenograft colon and pancreatic tumor models and hypothesize that, knockdown of D5D can reserve more DGLA in tumors to promote the formation of 8-HOA form COX-2 peroxidation to a threshold level, which could then suppress tumor growth and metastasis. To test our hypothesis, wild-type or D5D knockdown (via shRNA transfection) HCA-7 (human colon cancer cell line) and BxPC-3 (human pancreatic cancer cell line) were injected subcutaneously into the flank area of nude mice to establish tumor xenografts. The mice then received four-week treatment of vehicle control, DGLA, chemo-drugs or combination of DGLA and chemo-drugs. The tumor size was monitored and tumor tissues were collected followed by further analysis. Our results showed that D5D knockdown along with DGLA supplement inhibited growth and metastasis potential of colon and pancreatic xenograft tumors. A significant accumulation of 8-HOA was observed in D5D knockdown tumors after DGLA treatment, which resulted in inhibited HDAC activity and altered expression of various key proteins involved in cancer proliferation and migration, including p53, procaspase 9, cleaved PARP, Ki-67, E-cadherin and MMP-2. In addition, combination of our treatment strategy with chemo-drugs led to significantly improved anti-cancer effects in both colon and pancreatic tumors compared to chemotherapy alone. In conclusion, we demonstrated that the commonly overexpressed COX-2 in cancer can be taken advantage to control tumor growth and metastasis potential, which represents a paradigm shifting concept in contrast to the COX inhibition strategy.