Heteroepitaxial oxide thin films are usually grown on single crystal substrate which offer a similar lattice constant as the target material. In general, there are no substrates that are suitable for film fabrication and have a good lattice matching. In our previous report, we succeeded in fabricating a lattice constant tunable buffer by using a Ba1-xSrxTiO3 / BaTiO3 bilayer structure on SrTiO3. The in-plane lattice constant of the buffer layer can be tuned from 3.91 to 3.99 Å and the lattice constant is determined only by the Ba/Sr ratio in the Ba1-xSrxTiO3 layer. The buffer is suitable for growing both strain-free films and strained films. In this report we demonstrate the use of the lattice constant tunable buffer for strain control in SrRuO3 magnetic thin films. SrRuO3 has a magnetic anisotropy which changes under lattice strain. We show that the easy axis of magnetization is parallel to the film surface under tensile strain on a Ba0.5Sr0.5TiO3 buffer. The easy axis is perpendicular to the surface in compressively strained films. The tensile strain in a film also results in an increase of the ferromagnetic ordering temperature from a bulk value of 160 K to 164 K.