Crystal structure of the ammoniated alkali fulleride, (NH3)K3C60, was studied by high resolution x-ray diffraction experiments. We found a structural phase transition at low temperature. A superstructure appears below the transition temperature, T-s similar or equal to 150 K, and it originates from the orientational ordering of K-NH3 pairs in the octahedral site. Below T-s, K-NH3 pairs align in an antiferroelectric fashion, which is understood by electrostatic interaction between them. We also measured a deuteration effect to the transition. Similar transition was observed in (ND3)K3C60 and difference in T-s was very little.