We have studied the density functional theory of epitaxially strained ABO(3) (A = La, Y, Sc, B = Al, Ga) films on SrTiO3 (001) crystals with SrO- and TiO-terminations to investigate polar catastrophe induced by the epitaxial thin film growth. Interfacial polar nature of the strained films depends on the type of cations. While lanthanum based perovskite films produces polarity based on the ionic nature of interfacial atoms, the smaller cations show rather opposite polarities arising from progressive ferroelectric structural distortion acting opposite to the polar nature of cation ions.