Protein therapeutics have great potential in treating human disease, especially for those caused by alternations in the functions of intracellular proteins. However, clinical use of protein by intracellular delivery has been hampered by the instability due to proteins' physicochemical properties, and some barriers in the delivery pathway. This study was to prepare and test a novel intracellular protein delivery system - magnesium phosphate nanoparticles with cationic lipid coating for catalase intracellular delivery (LP MgP NP-CAT), and investigate whether it can release the encapsulated catalase to cytosol. LP MgP NP-CAT was designed, prepared and characterized, showing that it had an average diameter around 300 nm and zeta potential around +40mV. The pH - triggered catalase release from LP MgP NP-CAT was determined by a hydrogen peroxide degradation assay, where the concentration of remaining hydrogen peroxide was measured by UV-Vis spectroscopy, indicating catalase was released in response to the drop of pH, which was confirmed by the morphology change of LP MgP NP-CAT observed by transmission electron microscopy. The in vitro catalase release behavior was conducted on MCF-7 cells and EA.hy926 cells. LP MgP NP-CAT was delivered into MCF-7 cells and the release behavior was determined by the resultant resistance of the cells against hydrogen peroxide using MTS cell viability assay. The delivery of LP MgP NP-CAT into EA.hy926 cells was determined by the decrease of the reactive oxygen species level. Both of the studies showed that catalase was successfully delivered and released which is supported by the reduction of hydrogen peroxide.