Abstract: To study the characteristics of bed load transport during armor breakup and reformation, three groups of long-duration laboratory experiments were conducted without sediment supply. Each experiment group employed a stepwise increasing discharge. The bed materials used were composed of a fine fraction and a coarse fraction, with diameters of 1.22 mm and 3.74 mm, respectively. Accurate and high-resolution data of real-time bed load grain size and sediment transport rates were acquired from the experiments with the help of a newly designed sediment trap. Then, the evolution of the cumulative transported mass, bed-load transport rates and the grain size of the bed load were analyzed. The results show that the cumulative mass exhibits a power function relation with time and that there are turning points along the curves during armor breakup and reformation. The bed-load transport rates are highly unsteady, and the duration of the armor formation phase is 3.5 to 20.5 times longer than that of the breakup phase. The coarse fraction and transport rate of the bed load vary with time in a similar manner. When the transport rate peaks, the coarse fraction of the bed load is equal to the coarse fraction of the bed materials.