The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity polycrystalline silicon through a unidirectional solidification technique. In the solidification velocity range of 1.25 - 2.5 ×10-6m/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, in above the critical velocity around 40 ×10-6m/s, equiaxed structure appears. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 % to 80 % of the radius of the general two-dimensional nucleus. The reduction of the critical radius and undercooling on the reentrant corner could influence on the priority growth direction and the enlargement of the grain size.