In-situ tensile tests with specially designed SEM were conducted to trace the entire process of crack initiation and propagation till fracture in X80 pipeline steel. The mode of crack initiation induced by inclusions under tensile load was investigated. The results show that: (1) All the inclusions crazed and fracture was the typical tensile fracture, which indicates that the inclusions have little effect on the tensile properties of the X80 material under tensile load. (2) When the size of inclusion is larger than the critical size, crack initiated at inclusion/matrix interface first, and could easily propagate into matrix，but the long crack cannot form. When the size of inclusion is smaller than the critical size, cracks first formed in the inside of inclusions but not propagate into matrix. (3) If the area of cross section of round inclusion is less than 100μm2, crack initiated firstly in the inside of inclusion. If the area of cross section of round inclusion is larger than 100μm2, crack initiated firstly at the inclusion/matrix interface. For the rectangle inclusion, when its area of cross section is below 150μm2, cracks usually initiate in the center of inclusions.