The main objective of this study was to develop a polymeric drug delivery system for tamoxifen (TMX), intended to be injectable Eudragit® nanoparticles (NP) for breast cancer treatment. TMX-Eudragit-NP were characterized in terms of particle size, surface morphology, drug physical state by using photon correlation spectrometry (PCS), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The entrapment efficiency (%EE) and in vitro drug release studies were estimated spectrophotometrically by UV-vis technique. The cell toxicity assay was performed in 4T1 cell line using MTT test. TMX-Eudragit-NP showed a maximum entrapment efficient of 23%. The size measurements were compared with the empty nanoparticles and showed values of TMX-Eudragit-NP = 133 ± 30 nm nm, and empty- NP = 273± 50 nm. The zeta potential of particles was +65 and +38 mV for TMX-Eudragit-NP and empty-NP respectively. FTIR studies did not indicate changes in chemical structure or polymer-drug interaction. The cytotoxicity against the 4T1 cells was affected significantly by the released amount of TMX, while empty-NP exhibit no significant cytotoxicity against mouse breast cancer cells (4T1 cell line).