We have systematically investigated the magnetic properties of La- and Co-substituted SrFe12O19 using single crystals. By utilizing the traveling solvent floating zone technique, we have grown single crystals over a wide range of substitution content, and examined their saturation moments and magnetocrystalline anisotropy. With increasing content of La and Co, the saturation moment at 300 K monotonically increases. The increment of magnetocrystalline anisotropy is almost proportional to the content of Co. In addition, further La substitution also enhances magnetocrystalline anisotropy, which demonstrates that Fe2+ plays an important role to enhance magnetocrystalline anisotropy. We have analyzed magnetocrystalline anisotropy energy as a function of substitution content, and elucidate the effects of Co2+ and Fe2+.