There have been conflicting reports by researchers on the effect of aqueous leaf extract of Ocimum gratissimum (OG) on haematological parameters This study was carried out to evaluate the dose-dependent effect of OG leaf extract on some haematological parameters in wistar rats. Twenty-four male rats were randomly assigned into four groups of six rats each. Group 1 was a control while groups 2, 3 and 4 were test groups (T1 – T3) and were given 450mg/kg, 800mg/kg and 1800mg/kg respectively of OG leaf extract daily for 30 days. All rats had free access to water and rat chow. Results showed a significantly decreased RBC count in T2 (p<0.05) and T3 (p<0.01) compared with control. Platelet count and PCV were significantly decreased in T3 (p<0.05) compared with control. A significant increase in total WBC count was observed in T1 (p<0.05) and T2 (p<0.01) when compared with control. A significant reduction in neutrophil count in T1 (p<0.05), T2 (p< 0.05) and T3 (p<0.01) compared with control was observed. Lymphocyte count was significantly increased in T1 (p<0.05), T2 (p<0.01) and T3 (p<0.01) compared with control. There was no significant difference in MCV, MCH and MCHC in the different groups. In conclusion, aqueous leaf extract of Ocimum gratissimum caused reduction in RBC count, PCV, platelet count and neutrophil count but caused increases in total WBC count and lymphocyte count