A novel polar oxide of ZnSnO3 with LiNbO3-type structure has been investigated using first-principles density functional theory. The calculated pressure dependence of the phase stability in the ternary Zn2+-Sn4+-O2- system confirms the experimental results and detailed mechanism of the pressure-induced phase transition (see Fig.). High spontaneous polarization of 56.9 degrees C cm(-2) is calculated by the Berry-phase approach, and it is attributed to the large displacement of Zn2+ and its strong ionicity. Further improvement of the spontaneous polarization is suggested by enhancing the covalency of Sn4+ sites.