The effects of sputter growth conditions of Ru–SiO2 under-layer (hereafter referred as Rut layer) on the microstructure of this layer were studied. The effects of growth conditions of the Rut layer on the microstructure and magnetic properties of the Co72Pt28–SiO2 top layer were further investigated. Results show that increasing SiO2 content in the Rut layer deteriorates slightly the texture of Ru grains in this layer generally. The high-pressure (1.3 Pa) deposition can better protect the texture of Ru than low-pressure (0.4 Pa) deposition with SiO2 addition. Furthermore, high-pressure deposition of Rut layer with low SiO2 content produces better texture of Co72Pt28 than low pressure. The sizes of Ru grains and Co72Pt28 grains decrease with increasing SiO2 content. A high-pressure deposition of Rut layer further reduces the grain size of Co72Pt28. Both the coercivity and the remanence ratio of Co72Pt28 layer decrease when SiO2 content in the Rut layer exceeds 10 vol.%. High-pressure deposition of Rut layer results in a higher coercivity and remanence ratio of Co72Pt28 layer than those with low-pressure depositions. The above results show that the grain size of Co72Pt28 can be reduced without sacrificing its magnetic properties at small SiO2 content.