The efficient delivery of anticancer agents into tumor microenvironments is critical for the success of cancer therapies, but it is a prerequisite that drug carriers should overcome tumor vasculature and possess high drug contents. Here, we found that photoinduced inflammation response caused the migration of neutrophils into tumor microenvironments and neutrophils transported neutrophil-targeted nanoparticles (NPs) across the tumor blood barrier. The results showed that tumor delivery efficiencies of NPs were 5% ID/g, and they were independent of particle sizes (30-200 nm) and their doses (10-10 NPs). To efficiently deliver anticancer agents into tumors via neutrophils, we fabricated carrier-free paclitaxel nanocrystals (PTX NC). The results showed that neutrophil uptake of PTX NC did not impair neutrophil tumor infiltration, and the sustainable release of PTX from PTX NC in tumors was regulated by paclitaxel protein complexes, thus improving the mouse survival in two preclinical models. Our studies demonstrate that delivery of nanocrystal drugs via neutrophils is a promising method to effectively treat a wide range of cancers, and we have also identified a mechanism of drug release from neutrophils in tumors.