Cell membrane coated nanoparticles (NPs) have recently been recognized as attractive nanomedical tools because of their unique properties such as immune escape, long blood circulation time, specific molecular recognition and cell targeting. However, the integrity of the cell membrane coating on NPs, a key metrics related to the quality of these biomimetic-systems and their resulting biomedical function, has remained largely unexplored. Here, we report a fluorescence quenching assay to probe the integrity of cell membrane coating. In contradiction to the common assumption of perfect coating, we uncover that up to 90% of the biomimetic NPs are only partially coated. Using in vitro homologous targeting studies, we demonstrate that partially coated NPs could still be internalized by the target cells. By combining molecular simulations with experimental analysis, we further identify an endocytic entry mechanism for these NPs. We unravel that NPs with a high coating degree (≥50%) enter the cells individually, whereas the NPs with a low coating degree (<50%) need to aggregate together before internalization. This quantitative method and the fundamental understanding of how cell membrane coated NPs enter the cells will enhance the rational designing of biomimetic nanosystems and pave the way for more effective cancer nanomedicine.