Shake mixing has been widely used in cell culture. The mixing performance for shake mixing, however, has not been reported quantitatively. The mixing time, flow pattern and power consumption are measured in a vessel shaken in a horizontal circle. The minimum rotational shaking as circulating frequency N-C for complete mixing in a shaking vessel has been observed. This is referred teas ’the critical circulating frequency for complete mixing’. N-C was empirically correlated with the Reynolds number (Re = Nd-2/nu) and Froude number (Fr = (ND)-D-2/g), where D and d were the vessel diameter and the circulating diameter, respectively. The observed flow patterns were categorized into two types, i.e., the progressive wave type (N less than or equal to N-C or N greater than or equal to N-max,N-p) and the rotational flow type (N-C less than or equal to N less than or equal to N-max,N-P). Complete mixing was not achieved with the progressive wave flow, but it was achieved with the rotational flow in the following region. 0.135 Re--0.176 < Fr < 0.135 Re-0.166 (Re > 1) The power consumption in a shaking vessel was measured with a wattmeter. Power number N-P(= P/rho (ND5)-D-3) was correlated with Re, Fr and d/D, as N-P = 934Fr(3/2) Re--1/4 (d/D)(3/2) The mixing time during vessel shaking was slightly longer than with a rotating impeller at the same power consumption per unit volume.