A quasi one-dimensional (1D) compound, NaMn 2 O 4 , in which Mn 2 O 4 zigzag chains form a 1D channel along the b-axis and Na ions locate at the center of the channel, is thought to be a good Na ionic conductor. In order to study Na-ion diffusion, we have measured µ + SR spectra using a powder sample in the temperature range between 100 and 500 K. A diffusive behavior was clearly observed above 325 K. Assuming a thermal activate process for jump diffusion of Na-ion between two nearest neighboring sites, a self diffusion coefficient of Na ion (D Na) and its activation energy (E a) were estimated as D Na = (3.1 ± 0.2) × 10 −11 cm 2 /s at 350 K and E a = 180(9) meV.