The mortality of lung cancer in Gejiu has reached the peak in the world. The indoor radon levels in Gejiu are measured by SSNTD. The results indicate that a higher indoor radon level widely exists in Gejiu area; the radon level in houses situated in geologic fault zone is 6 times as high as 2 km from fault zone; the radon level in soil in faults is 6~8 times as high as 1 km from faults; the radon level of the cancer patients' houses is 1.45 times as high as healths'. The relationship between indoor radon level and incidence of lung cancer is given for the first time.