The influence of carbide morphology on the deformation and fracture mechanisms of as-received and complete spheroidization 14Cr1MoR steel was investigated using an in situ scanning electron microscope (SEM) under tension testing. During spheroidization damage, the carbide morphology changed from the original lamellar cementite present in pearlite to granular M23C6 carbide, which was concentrated along the ferrite grain boundaries. The yield strength and tensile strength of 14Cr1MoR steel decreased with the increasing degree of spheroidization damage. In situ SEM observations revealed that the deformation and crack initiation started from the ferrite matrix in both as-received and completely spheroidization-damaged 14Cr1MoR steel samples. However, the extension of slip bands and crack propagation behavior of both samples were different during the in situ tensile process, which could be ascribed to the difference in carbide morphology. In the as-received 14Cr1MoR steel sample, hard and brittle lamellar pearlite resulted in high-strength ferrite/ pearlite boundaries, which inhibited the movement of slip bands. With further deformation, the concentration of stress at the crack tip resulted in the emergence and propagation of cracks along the ferrite/pearlite boundaries. In the case of the completely spheroidized 14Cr1MoR steel sample, slip bands bypassed the grain boundary carbide and continuously expanded into the neighboring ferrite grain. In addition, micro-voids and fractures of grain boundary carbides were observed due to the large stress concentration at the front of crack tip. Then, the micro-voids connected with the main crack to complete the crack propagation behavior. The morphological changes of carbides deteriorated the mechanical properties and altered the fracture behavior of 14Cr1MoR steel. It is worth noting that the fracture surface morphology of 14Cr1MoR steel changed from a combination of lamellar fracture and dimpled morphology to a completely dimples-dominated morphology after spheroidization.