In this study, ten dogs, 6-8 months aged, were used. All dogs were fed normal diet plus cholesterol powder for ten days. At the end of the period, the means plasma cholesterol levels increased from 64. 60 to 230. 20 mg/ dl. Hypercholesterolemia didn't cause deformation in erytrocytes shapes. Then, ten blood samples taken from all the dogs were divided equally into two groups as control and experimental. Two hours after as in vitro 0. 2 µl of onion oil addition to each 10 ml of blood sample in experimental group, plasma cholesterol levels in experimental and control groups decreased from 238. 20, 180. 40 and 171. 60 mg/dl respectively. Plasma glucose levels in experimental group decreased significatly as compared to control group after onion oil administration (P<0. 05). After in vitro onion oil administration to experimental group, erytrocytes shapes showed deformation and crenation due to Heinz body formation resulting obviously falling in erytrocytes count, haemoglobine amount and haematocrit values in comparison with control group. Consequently, in this investigation, it was determined that onion oil had no effect on plasma cholesterol levels whereas reduced plasma glucose levels and had haemolytic effect on erytrocytes.