Objective To evaluate the capability of 18 F-FLT uptake and investigate the early radiation response of human colorectal cancer cells HCT116 exposed to 6 MV X-rays.Methods 3.7 kBq 18F-FLT was added to HCT116 cells with different cell numbers (1.0 × 105-1.5 × 106) and cultured with different times (36,60,84 h).The 18F-FLT uptake rate was measured with a γ-counter after exposed to different does of 6 MV X-rays (0,2,4,6,8 Gy) after 24,48,and 72 h of irradiation.Then the cell uptake inhibition rate,cell proliferation,and cell cycle phase were measured.Results The uptake rate of 18F-FLT in HCT116 was (18.97 ± 1.16)％.The 18F-FLT uptake inhibition rates at 24 h after different does of irradiation (2,4,6,8 Gy) were (32.10±0.02)％,(54.46 ±0.04)％,(62.74 ±0.04)％,and (65.81 ±4.81)％,respectively,which was positively correlated with radiation dose.Conclusions The 18F-FLT uptake rate of human colorectal cancer HCT116 cells could be used to evaluate the early radiation response.