A simple theoretical model for the origin of spontaneous polarization in nanocrystals is developed. We propose that the origin of the spontaneous polarization is in the boundary layer between "cap" and the nanocrystal, and the internal electric field in the dot is due to the piezoelectric effect caused by the strain existing in the interface region of materials with different lattice constants. The model, based on spherical rotation symmetry without inversion [SO(3)], employs a distribution of polarization with symmetry which is a subgroup of SO(3) consistent with the hexagonal structure of wurtzite structure. We predict the internal electric field and compare with experimental data.