Abstract: The method of geogas measurement was used to detect concealed fault at the site of Erlangshan tunnel. Through processing and comparative analysis of data of five typical sections, the paper determined effectively the existence, the position of outcrops, the dimension and geometrical features of fault, and analyzed the abnormal characteristics of geogas elements of concealed fault. The results show that there exists geogas anomaly along the survey line above the inferred fault at the site of Erlangshan tunnel,and the anomaly has good correspondency to structure. Elements closely related to diagenesis such as La, Ce, Nd, Sm, y, Cd, Cu, Sr, Pb have high sensibility and evident peak, can well reflect the existence of concealed fault. There exists certain relation between the amplitude of geogas anomaly, the cutting depth of fault and the activity of fault.