Poly(ethylene glycol) functionalized carbonnanotubes (CNTs-PEG) have attracted great researchinterest due to their good biocompatibility and wideapplications in biomedical areas, such as drug delivery,bioimaging and photothermal therapy. The biodistributionstudies are fundamental and essential to the biomedicalapplications and toxicological evaluations of CNTs-PEG,while isotopic labeling is the most adopted method for thequantitation of CNTs in vivo. Herein, we studied the biodistributionof hydroxyl terminated CNTs-PEG (CNTs-PEG-OH) in mice using 125I-labeling technique. The bloodconcentrations and accumulation levels of CNTs-PEG-OHin different organs were quantitatively analyzed bydetecting the radioactivity of 125I labeled on CNTs-PEGOH.The results indicated that CNTs-PEG-OH werequickly cleared from blood circulation and distributed tothe entire body, except brain. CNTs-PEG-OH were mainlysequestrated by liver after intravenous injection. Thehepatic accumulation decreased along with time elapse.Prolonging the PEG chains led to the decrease of hepaticaccumulation, while the biodistribution pattern was similar.The implications to the biomedical applications and safetyevaluations of CNTs-PEG are discussed.