This article deals with the characterization of the mechanical behaviors of spheroidized carbon steels, which contain ferrite and spheroidal cementite. The aim of the study is to identify the deformation behaviors of cementite particles by x-ray diffraction and to correlate them to the residual stress state evolution. The stress state evolutions for both phases during an in situ 4-point bending test and after a uniaxial tensile test that have been well followed by x-ray diffraction. After each tensile loading, optical observation is made to correlate the determined residual stress state with the microstructure. The coupling of techniques helps us to characterize the deformation evolution of spheroidal cementite in the carbon steel: at the initial state, the cementite shows the same elastic behavior as the ferrite; at the next state, the ferrite begins to deform plastically but the cementite deforms elastically; then, plastic relaxation occurs in the ferrite while the cementite still deforms elastically until finally breaking.