Background and objectives the 5nucleotidase (5NT) is one of hydrolytic enzymes present in different organs which catalyze hydrolysis of 5 nucleotides to ribonucleosides and phosphate. Malondialdehyde (MDA) is the end product of lipid peroxidation by oxidative stress (free radicals). The aim of present study was to measure the serum activity of 5NT, and MDA concentration in breast tumors. Material and method A prospective study was carried out from May to December 2013 by clinical biochemistry department in College of Medicine Hawler Medical University on (30) healthy female individuals, (group 1) and (30) females with breast tumor (group 2). Results The mean value of serum MDA was significantly higher in females with breast tumors (group 2), than that of healthy female individuals, (group 1) (p<0.01), and serum5NT activity was significantly higher in females with breast tumors (group2), than that of healthy individuals (group 1) (p<0.01). Conclusion Based on findings of the present study it can be concluded that breast tumors can cause release of the enzyme 5NT from tumor cells, and lipid peroxidation by reactive oxygen species (ROS), which cause elevation of MDA.