Experimental investigations on the mixing behavior of a spherical particle in a spout-fluid bed with a cross section of 0.1 m×0.03 m and height of 0.5 m were carried out. The image processing technology and bed-frozen method were employed to measure the axial and radial concentration of the tracer at different times. Four various initial conditions (top, side, bottom and middle loading) were used to examine the influence of the tracer location on the mixing. In addition, the underlying mixing mechanisms were preliminarily discussed in terms of the evolution of the mixing index. It is shown that the initial tracer location affects the initial mixing rate but not the final mixing degree. The top and bottom loading cases characterised by dominant axial mixing perform significantly better than the side loading case characterised by dominant radial mixing. Due to the axial rapid convection and radial intense diffusion, the mixing speed for a middle loading case is much faster compared to other cases. The results also demonstrate that the convection caused by the circulation and diffusion among the circulation rolls are two main mixing mechanisms in a spout-fluid bed.