PEGylated particles are considered to be macrophage-resistant in blood circulation following intravenous injection. Therefore, drug-loaded particles in blood stream can extend its lifetime. In this study, we utilized PEG-PCL block copolymers designed to avoid attacking from macrophages. We used MTT assay to evaluate the cytotoxicity of nanoparticles formed by self-assembly of the block copolymers. The flow cytometry was utilized to analyze the interaction of the polymeric nanoparticles with a macrophage cell line (RAW264.7). It was found that the cell granularity did not change significantly in PEG-PCL block copolymer yet increased substantially in PCL-PAA (non PEG grafted copolymer). The preliminary results in this study suggested that the macrophage cells were not activated by PEG-PCL nanoparticles.