Stress evaluation in polymeric materials is important in order to not only spot danger in them before serious failure, but also precisely interpret the destructive mechanism, which can improve the lifetime and durability of polymeric materials. Here, we are able to visualize stress by color changes, as well as quantitatively estimate the stress in situ, in segmented polyurethane elastomers with diarylbibenzofuranone-based dynamic covalent mechanophores. We prepared films of the segmented polyurethanes, in which the mechanophores were incorporated in the soft segments, and efficiently activated them by mechanical force. Cleavage of the mechanophores during uniaxial elongation and their recovery after the removal of the stress were quantitatively evaluated by in situ electron paramagnetic resonance measurements, accompanied by drastic color changes.