Nonsteroidal antiinflammatory drugs (NSAIDs) exert their antiinflammatory, analgesic and antipyretic effects by reducing synthesis of prostaglandins via cyclooxygenase (COX) inhibition. Despite the extensive applications of NSAIDs, both in human and in veterinary medicine, the use of these drugs is limited due to their sever adverse effects especially on gastric mucosa. Classic studies attribute the therapeutic and the adverse effects of NSAIDs to the inhibition of COX-2 and COX-1 respectively. Celecoxib, a COX-2 specific drug, has caused less gastric effects compared to the classic non-specific NSAIDs. However, to the best of our knowledge, gastric effects of this drug have not been studied yet. In this research, dogs (mixed Iranian breed) in groups of 5, received either celecoxib (3mg/kg; twice a day) or placebo (control) orally for 14 days. The haemogram was studies during the treatment and 3d after cessation of celecoxib (day 17). The lesions of gastric mucosa were also graded at days 0, 7 and 14 using standard gastroscopy according to the modified method of Murtaugh and his colleagues. The haemogram results (WBC, RBC and platelet counts as well as the levels of PCV, hemoglobin, MCV, MCH and MCHC) were not statistically different between the two groups. However, celecoxib increased the bleeding time 3d after cessation of the treatment. Besides, significant lesions were observed on gastric mucosa of celecoxib receiving dogs especially at day 14. These results suggest that celecoxib, due to its potential sever gastric effects, and should be used cautiously in dogs.