A nanocarrier, lactosome, which is composed of poly(L-lactic acid)-block-poly(sarcosine), as a contrast agent for the liver tumor imaging was examined using the near infrared fluorescence (NIRF) optical imaging technique. Lactosome labeled with indocyanine green (ICG) showed a high escape ability from the reticulo-endothelial system (RES). Lactosome was found to be stable in a blood circulation, and gradually accumulated specifically at a model liver tumor site, which was obtained by graft of HepG2/EF-Luc cells at a mouse liver. The high tumor/liver imaging ratio is due to the enhanced permeation and retention (EPR) effect of lactosome. The fluorescence intensity at the tumor site was correlated with the degree of malignancy. Tumor imaging using lactosome as a nanocarrier is therefore a potential candidate for a facile and general tumor imaging technique.