Power characteristic of a rotationally reciprocating impeller was investigated in view of torque variation using Newtonian and viscoelastic fluids. The power curve for both fluids showed good agreement by the introduction of the appropriate definition of Reynolds number based on Metzner-Otto concept. The raw date of torque variation was decomposed into sinusoidal waves having various frequencies (higher harmonics) with the help of FFT. The phase difference of the first harmonic against reciprocating motion showed that the torque variation was significantly affected by elasticity at low Reynolds numbers, where the dimensionless intensity was inversely decreased against Reynolds number similar with power number. It is found for viscoelastic fluid that the relative intensity and phase difference of the third harmonic were roughly constant except for viscous dominant region. Since those information of higher harmonics are cancelled in the calculation of time-averaged power consumption, it is found that the frequency analysis of torque variation is useful to elucidate dominant factors on torque variation and probably on fluid flow generated by a rotationally reciprocating impeller. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.