Recently, the development of high-speed heavy-duty laser printers is rapidly shifting from a conventional test-based approach to a virtual design procedure using numerical analyses because the later approach is more time efficient. The authors have developed a three-dimensional (3-D) analysis for a roll fuser design.Peculiar difficulties in a high-speed heavy-duty roll fuser design are due to the nature of significant heat supply in a nip region during short period and an insufficiently short period for temperature recovery in an outer nip region. Therefore, the only use of the 3-D analysis gives little reliability growth.The authors analyze the thermal mechanisms, and then identify problems giving the difficulties for the fuser design. The problems are significant temperature gradient occurrences, and their rapid and complex changes in the fuser, toner and paper in micro scale distance in the nip and the outer nip regions during printing. For instance, even in the outer nip region, several degrees to several ten degrees Celsius temperature difference occurs in several hundred microns below a fuser roll surface; in consequence reduces reliability due to high temperature occurrence on an interface between a surface layer and a core metal in the heat roll.Combining the 3-D analysis and the analysis for the thermal mechanisms, the authors obtain knowledge for the robust fusing system design in the high-speed heavy-duty laser printers.