Multi-walled carbon nanotubes (MWCNTs) have been proposed for various biomedical applications which require their systemic administration. Therefore, the present study was undertaken to investigate the effects of carboxyl group functionalized multi-walled carbon nanotubes (COOH-MWCNT) on hematological parameters and morphology of erythrocytes of Wistar rat. Forty eight adult rats of both the sexes were used for the study. Rats of experimental groups were intraperitoneally injected with 0.5 ml of COOH-MWCNT suspension in double distilled water containing 1% tween-80 at a dose of 2.5, 5 and 10 mg/kg body weight on alternate days. Rats of control group received 0.5 ml of vehicle. Half of the animals of each group were sacrificed after 30 days and remaining half after 60 days of treatment. The results of present study indicate a significant reduction in the number of total erythrocytes, hemoglobin levels and hematocrit values while significant increment in the number of leukocytes in MWCNT administered rats at both the durations of the study as compared to control rats. Mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) remain significantly unaltered while Mean corpuscular volume (MCV) decreased in high dose group only at 60 days of treatment duration. Differential leukocyte count revealed a decrease in the percentage of neutrophils while increase in the percentage of monocytes and lymphocytes. Scanning electron micrograph of blood of treated rats showed presence of echinocytes and acanthocytes. Thus, results of our study suggest that intraperitoneal administration of COOH-MWCNTs have adverse effect on the hematological parameters of rats.