Flexible heat- and steam-generating sheets are promising medical devices for human healthcare, and oxidation of iron powder in sheets has potential applications for this purpose. Hence, the preparation conditions of composite sheets containing iron powder, cellulose fiber and activated carbon powder using techniques in the papermaking process were investigated to increase the iron powder content as much as possible and improve retention ratios of iron powder in the sheets. When unfibrillated cellulose fibers were used without any retention aids, the total retention ratios of the components were less than 40%. By contrast, when highly fibrillated cellulose fibers were used in combination with a cationic/anionic dual polymer system, the total retention ratios at a basis weight of 217 g m(-2) reached about 90%. Moreover, iron powder aggregates in the sheet surfaces as well as throughout the thickness of the sheet, and higher tensile and internal strengths were obtained by using highly fibrillated cellulose fibers.