Abstract: A portion (0.5 mL) of pretreated human urine was taken and added with isotopic internal standard. It was purified by SPE and the eluate was centrifuged in vaccum to dryness. The residue was taken up with 0.2 mL of water. 8-Hydroxy-2'-deoxyguanosine (8-OHdG), a symbolizing product of oxidation damaging of DNA, was determined by UHPLC-MS/MS. Urine samples were taken from 65 healthy (including 21 smokers and 44 non-smokers) and 61 patients suffering from colorectal cancer (including 17 smokers and 44 non-smokers), and urinary 8-OHdG in these samples were determined by the proposed method. It was found that: 1 concentration levels of urinary 8-OHdG for smokers (both of healthy persons and patients) were significantly higher than those for non-smokers; and 2 concentration levels of urinary 8-OHdG for the patients (both non-smokers and smokers) were also significantly higher than the healthy persons (both smokers and non-smokers). It was concluded that smoking, as a risk factor, promoted oxidative damage of DNA and led to occurrence of colorectal cancer.