The targeting of drug nanocrystals to the malignant and metastatic components is a potential strategy for tumor therapy. Compared with nanocarrier drugs, nanocrystals have many excellent advantages such as relatively simple preparation process, high drug loading, and good physical stability. In this chapter, the application of nanocrystal technology in the treatment of tumors is reviewed and discussed. First, the preparation methods of nanocrystals are summarized. Second, administration routes of drug nanocrystals are reviewed. Third, the status of drug nanocrystals in the market and clinical trials is summarized. Fourth, recent researches on drug nanocrystals in the tumor target delivery are discussed. Finally, the application of drug nanocrystals for the treatment of tumors is summarized and reviewed. This chapter provides useful information to understand the research and development of drug nanocrystals for the treatment of tumors.