Nickel based superalloy AM3 was prepared in high thermal gradient directional solidification apparatus with a seeding technique. Off-axial seeds were used in the experiments to investigate the nucleation and competitive growth of stray grains. The results indicate that the local undercooling caused by off-axial dendrites could result in stray grains formation. The grains with dendrite direction close to the heat flow direction can be overgrown by those grains which have larger deviation from the heat flow direction. The results reveal that the competitive growth of stray grains depends upon the synthetical effects of the comparative position of the dendrites, the angles between the dendritic direction of the grains and the heat flow direction, and the orientations of the crystal grains.