We have for the first time investigated the gas-solid fluidization behavior in size-reduced beds called micro fluidized bed (Liu. X et al. Chem. Eng. J., 2008, 137: 302-307), but up to now there is not yet any clear definition for the micro fluidized beds (MFBs). This study intends to characterize MFBs in terms of gas back-mixing. The residence time distribution (RTD) and extent of back-mixing of gas were investigated in beds with inner diameters of up to 21 mm for particles of FCC, glass beads and silica sand. The RTD curves of gas, shown as E(t) versus time t, in such beds were determined on the basis of axial dispersion model to obtain the mean residence time (t) over bar, variance sigma(2)(t) and peak height of a given E(t) curve. In terms of these parameters the degree of gas backmixing and its variation were evaluated with respect to particles, bed diameter (D), superficial gas velocity (U-g) and static particle bed height (H-g). It was found that the RTD of gas is subject to a unique correlation between the height of E(t) peak and the variance sigma(2)(t), which makes it clear that the gas flow in an MFB has limited gas dispersion and is highly approaching to a plug flow if sigma(2)(t) is below 0.25 or the height of E(t) peak is larger than 1.0. This refers to the feature that is most desired for an MFB and can thus be the criterion for defining MFB.