Fabrication of near-infrared (NIR) fluorescence nanoprobes (NPs) has drawn great attention due to their potential as highly sensitive optical probes for in vivo tumor molecular imaging.Herein, a facile strategy was reported for the preparation of ultrasmall (<20 nm) indocyanine green (ICG)-containing NIR fluorescence NPs using self-assembly chemistry between folate (FA) modified water-soluble chitosan (WCS) and ICG.The ICG-containing NPs exhibit characteristics of crystalline solid, good photostability, lower cytotoxicity and high tumor targeting ability for in vitro cell imaging.We also explored this system for in vivo tumor molecular imaging.