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Hydrous ruthenium oxide (RuO 2) has been considered a potential electrode for supercapacitors. However, RuO 2 is not easy to be hydrous, which impedes advancement in RuO 2 -based supercapacitors. Here, strontium cation (Sr2+) – with a lower oxidation state than Ru4+ – doping effects on improving the capability of RuO 2 to be hydrous are reported. The Sr2+ doping in the RuO 2 was simply possible using electrospinning and subsequent calcination, affecting hydrous properties and promoting miniaturization of nanograins in one-dimensional (1D) nanofibers. The compositions of hydrous Sr2+-doped RuO 2 (H-Sr x Ru 1−x O 2−x) nanofibers were optimized considering correlations between synthesis temperature and degree of hydration. It was found that the H-Sr 0.2 Ru 0.8 O 1.8 nanofibers synthesized at 400 °C were hydrous compared with the pristine RuO 2 and H-Sr 0.1 Ru 0.9 O 1.9 nanofibers. The H-Sr 0.2 Ru 0.8 O 1.8 nanofibers (400) could possess the smallest nanocrystallites, increasing electrochemically active surface area and satisfying specific capacitance of 612.6 A/g at a scan rate of 50 mV s–1. • A Sr2+-doped RuO 2 nanofibers were successfully fabricated by electrospinning and calcination. • Sr2+ doping enabled RuO 2 nanofibers to be hydrous and to possess miniaturized nanograins. • Sr2+ doping in RuO 2 was optimized for annealing temperature and composition. • The highly hydrous Sr 0.2 Ru 1.8 O 1.8 nanofibers boost the capacitance performance. [ABSTRACT FROM AUTHOR]