We prepared fluorescent cyanine 5.5 dye and cyclo(arginine-glycineaspartic acid-phenylalanine-lysine) ligand-modified polyethylene glycol-poly(lactic-co-glycolic acid) (Cy 5.5-cRGDfK-PEG-PLGA) NPs with different ligand concentrations and investigated their cancer-targeting abilities in vivo. In vivo, NPs with 6 w/w% cRGDfK concentration showed prolonged uptake by the cancer tissue during the 7-day test period. These results suggest that Cy 5.5-cRGDfK6-PEG-PLGA NPs could serve as valuable drug carriers and diagnostic agents for the clinical treatment and targeting of solid cancers. ** This work was supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (20003560 and 20004627) and by the National Research Foundation of Korea (NRF) (2017R1D1A1B03033195) of Korea.