Chitosan and its derivatives have been widely used for various biomedical applications because of their unique chemical and biological characters. The amine groups in the backbone of chitosan allow chemical modification to change the physical properties of chitosan. Based on hydrophobic or charge interactions with this chitosan polymer backbone, stable self-assembled nanoparticles can be fabricated in aqueous condition. These nanosized structures enable intravenous injection and show large accumulation in tumor tissue, indicating great potential in imaging and drug delivery. The main objective of this review is to introduce various chitosan nanoparticles and their recent applications for tumor diagnosis and therapy.