Lateral mixing of solids in a high-density riser is investigated by a phosphor tracer method. The lateral solids dispersion is determined by measuring the solids RTD at different radii using point sources for tracer injection. A two-dimensional dispersion model describes the measured RTD curves satisfactorily. The cross-sectional average solids fraction has the dominant effect on the lateral solids mixing in the high-density riser. The lateral Peclet number increases exponentially with the solids fraction and is not sensitive to gas velocity and solids circulation rate. The lateral Peclet number in high-density riser is over 1200, indicating that the lateral mass transfer is a limitation factor for the application of high-density riser as a high efficient reactor. Correlation of the lateral Peclet numbers fits the experimental data well. When swirling air is introduced into the riser, the lateral Peclet number decreases and the lateral solids mixing is intensified by the swirling flow.