The present study aims to evaluate the cytotoxicity and antitumor activity of the ethanolic extract ofleaves of Syzygium aqueum (EESA) on K562 tumor cell line (chronic myeloid leukemia) andtransplantable tumor Dalton's ascites lymphoma (DAL). In-vitro cytotoxicity on K562 tumor cell linewas evaluated using micro-culture tetrazolium (MTT) assay. In-vivo antitumor activity was studied onDAL tumor bearing mice. Activity was assessed by monitoring the mean survival time, tumor cellcount, percentage increase in body weight and effect on haematological parameters like red blood cell(RBC) count, haemoglobin content (Hb), white blood cell (WBC) count, Differential cell (DC) count.Histopathological studies were also evaluated during this study. EESA exhibit potent in-vitro cytotoxicity activity against K562 tumor cell line. The result also showed an increased life spanand decreased tumor cell count in S.aqueum treated animals. The hematological studies revealed thatthe Hb count decreased in DAL bearing mice, whereas S.aqueum treated mice showed an increase inthe Hb or near to normal levels and other hematological parameters such as RBC, WBC was alsosignificantly reduced. Histopathalogical studies evaluated the destruction of tumor cells by showingcell wall destruction, membrane blebbing, degradation, vacuolated cytoplasm. The results suggestedthat the EESA exhibited cytotoxicity in dose dependent and it showed significant antitumor activityon DAL bearing mice.