One of the most crucial nanomaterials used in applications of biomedicine is Multi-Walled Carbon nanotubes (MWCNTs). It is applicable in delivering of different drugs to the tumor target sites, including doxorubicin (DOX). This is because the functionalized MWCNTs can penetrate membranes with low cytotoxicity. In this study, MWCNTs were oxidized (Ox-MWCNTs) to improve their solubility and biocompatibility. Their abilities to encapsulate DOX were tested (Ox-MWCNT/DOX). These particles were then characterized using FTIR, TEM, SEM and zeta size. Their antitumor activities against in-vitroMCF-7 and NBL cells were estimated. The results of FTIR confirmed the addition of carboxyl groups to the MWCNTs followed by encapsulation of DOX.Their shape was tubular with open ends using TEM. From their surface view, SEM showed the tubes as hollow spheres with interconnected structure. Higher releasing rate of DOX at pH 5.8 comparing with pH 7.4 was observed. Treating of human breast cancer cells(MCF-7) with Ox-MWCNT/DOX resulted in cellular viabilities of 96±0.37, 87±0.7, 34.7±0.75, 14.3±1 and 4.2±0.5 % at doses equivalent to 0.1, 1, 10, 100 and 1000µg/ml free DOX, respectively. Moreover, higher viabilities (99.8±0.13, 99.54±0.69, 77.42±1.13, 62.48±0.1 and 55.3±1.25%) with lower cytotoxicities of normal liver cells(NBL) were observed under the same treatment protocol. Therefore, the investigated loading protocol may be a promising one for the treatment of such tumor type after further studies in the future.