Semisolid slurries with non-dendritic microstructure can be produced by solidification under forced convection. Dendrite fragmentation is a commonly accepted mechanism for shearing semisolid slurry. However, it requires from experimental observation to confirm the fragmentation of dendrite arms occurring during shearing process. Experiments are undertaken to investigate the dendrite fragmentation in Sn-15wt% Pb alloy under shearing by virtue of a twin-screw extruder together with the wetting and penetration of pure tin grain boundaries submerged and sheared in the semisolid slurry of Sn-15wt% Pb alloy. It is found that the primary dendrites can be fragmented rapidly into small particles and there are penetration grooves existing in the partially fragmented dendrites. Such grooves are similar to that found at the grain boundaries in pre-cast pure tin polycrystals, which have been submerged and sheared in the semisolid slurry of Sn-15wt% Pb alloy. (C) 2003 Kluwer Academic Publishers.