Abstract: To investigate the effect of travelling electromagnetic field on dendrite fragmentation in solidification process, the 0.22 wt%~0.34 wt%C steel was tested for its solidifying behavior in travelling electromagnetic field. Observing the solidification structures of casting blanks under different magnetic flux density, the distributions of secondary dendrite arm spacing were investigated and the functional relationship between secondary dendrite arm spacing and cooling rate in solidification was discussed, as well as the effect of travelling electromagnetic field on the columnar-to-equiaxed transition (CET) position. The solidification rate and solid fraction when CET occurs were calculated. The critical conditions for dendrite fragmentation in travelling electromagnetic field is derived and verified by the testing results of low carbon steel. Thus, the relationship between the average velocity of the liquid-phase and the critical solid volume fraction when CET occurs was obtained.