Image gloss in electrophotographic copiers and printers is closely related to the image structures, which are spatial structures of melted toner layers on media. Therefore, in order to design the image gloss when copiers and printers are developed, it is necessary to clarify relations between the image gloss and the image structures, and relations between the image structures and system parameters. We clarified the relations between the image gloss and the image structures by constructing a relation model between the image gloss and characteristic indices of two image structures that crucially affect the image gloss in our previous report. However, with respect to clarifying the relations between the image structures and the system parameters, analysis on the melting deformation behavior of toner particles is the most important problem to be solved. In this report we newly developed a visualization measurement method for melting deformation of toner particles in a fusing nip. The method enables us to directly observe the behavior of toner particles in the fusing nip under the condition simulating time variation of temperature and pressure in the actual fusing system. By using the method, dependences of the deformation behavior of the toners in the fusing nip on the system parameters were analyzed. As a result, it was found that the toner particles are deformed completely in the fusing nip. We also clarified the effect on the deformation of toner particles in the fusing nip caused by temperature of a heating unit and that of a pressurizing unit. Considering the results, it was found that the restoring deformation of the toner after the fusing nip caused by residual stress stored inside the toner particle significantly affects the image structures and image gloss after fusing.