Ni-based single crystal (SX) superalloys are widely used in key hot section parts of advanced aero engine and industrial gas turbines (IGTs) because of their superior mechanical performance at high temperature. During directional solidification process of SX blades, high angle grain boundaries that degrade the creep and fatigue properties significantly might be highly likely to occur in the platform region, thus the analysis of multi-influencing factors, such as platform dimension, withdrawal rate and seed orientation, need to be studied. However, most of these works are conducted from the perspective of heterogeneous nucleation induced by the extreme concave shape of liquid-solid interface, and there is rare report concerning that whether dendrite deformation could also induce the high angle grain boundaries. Therefore, in the present work, the SX castings with three platforms were directionally solidified in a Bridgman-furnace, to investigate dendrite growth and the associated orientation evolution. It was observed that the whole platform consisted of three types of regions: blade body, secondary dendrite spread zone, and the "circuit-like" dendrite growth zone. The convergent boundaries of dendrite arms (CBDAs) were formed between secondary dendrite spread zone and the "circuit-like" dendrite growth zone. With increasing withdrawal rate, the area of "circuit-like" dendrite growth zone was increased, and the area of secondary dendrite spread zone was reduced. Moreover, the monotonically increased misorientation angle along CBDAs as a result of the dendrite deformation around platform edge was identified. As withdrawal rate increased, the misorientation angle along CBDAs was increased, and thus the tendency of high angle grain boundary formation on the CBDAs was enhanced. Unlike the nearly constant misorientation angle of the high angle grain boundary between grains and the low angle grain boundary between subgrains, the misorientation angle of the high angle grain boundary and low angle grain boundary formed on the CBDAs varied regularly.