The present study aimed to determine the effects of high dietary cholesterol (1%) and oleic acid rich oil (olive oil 5%) and a mixture of both on biochemical and haematological traits in chickens. The data obtained revealed that the feeding of olive oil alone resulted in significant increase in total serum globulins and gamma-globulins. The concentration of serum calcium orphosphorous was significantly decreased while that of magnesium was significantly increased in cholesterol - fed birds. On the other hand, serum triacylglycerol (TAG); total serum cholesterol, low density lipoprotein cholesterol (LDLc) and very low density lipoprotein cholesterol (VLDLC) were significantly increased in birds kept on high dietary cholesterol, meanwhile, the values of high density lipoprotein cholesterol (HDLC) was significantly decreased. These data were reversed completely in olive oil - fed birds Results also showed a non significant increase in values of lipid peroxidation and hydroxyl radical production in cholesterol – fed group, whereas the feeding of olive oil resulted in their significant decrease. The values of glutathione -S- transferase (GST) and reduced glutathione (GSH) showed a significant increase in hemolysate of olive oil fed birds, but they showed a non significant decrease in cholesterol -fed group. Concerning the blood picture, the cholesterol administration significantly decreased red blood cells (RBCs) count, packed cell volume (PCV) and haemoglobin (Hb) concentration, while the olive oil feeding led to significant increase in RBCs, white blood cells (WBCs) counts, PCV, haemoglobin concentration and lymphocyte percentage. In conclusion, the addition of olive oil to cholesterol containing feed, nearly counteract most of the undesirable effects resulted from cholesterol feeding.