We investigated the size effect on ferromagnetic resonance (FMR) in a submicron-wide single permalloy bar. The resonant frequency markedly increased with decreasing bar width to less than 1 m, since the demagnetizing field is effectively modified by changing the bar width even in thin films. The resonant frequency difference between 100- and 1000-nm-wide bars was over 4 GHz in the absence of a magnetic field. This characteristic is promising for practical microwave devices because the desired resonant frequency can be obtained simply by varying the width of narrow ferromagnetic bars so that it is not necessary to change the material or magnetic field. © 2013 The Japan Society of Applied Physics.