We have investigated electronic and magnetic properties of ultrathin SrRuO 3 (SRO) films grown on (111) SrTiO 3 substrates using the ab initio electronic structure calculations. Ru-terminated SRO (111) films suffer from strong surface atomic relaxations, while SrO 3 -terminated ones preserve the surface structure of ideal perovskites. Both Ru-and SrO 3 -terminated SRO (111) films show unexpected interlayer antiferromagnetic (AFM) structures at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO 3 -termination case, the half-metallic nature emerges despite the interlayer AFM structure. Upon reducing the thickness, the collapsing behavior of magnetic anisotropy from out-of-plane to in-plane easy axis is found to occur for the Ru-termination case, which, however, does not pertain to the SrO 3 -termination case.