Abstract: In situ tensile tests in scanning electron microscope (SEM) were carried out using single-edge notched specimens of no larger than 0.5 mm in thickness made of CL60 wheel steel with or without hydrogen charging. Observations under optical microscope (OM) were also done to a wedge opening-loading (WOL) specimen with a thickness of 30 mm which was precracked. The result shows that, for the thinner specimen, plastic deformation occurs firstly in proeutectoid ferrite and then grows along the prior austenite grain boundary, and then, a microcrack initiates and propagates preferentially at the interface of proeutectoid ferrite and pearlite colony. The process of deformation and fracture is almost same each other for both specimens with and without hydrogen, except that the microcrack initiates more frequently by way of inclusion separating and shelling from matrix for the specimen with hydrogen. The fracture surface fractograph of the thinner specimen is ductile fracture. However, for the thicker specimen (WOL), plastic deformation and cracking mainly occurs in the pearlite colony, which results in cleavage fracture.