Co-Pt-B-O alloy films show the magnetic properties of saturation magnetic induction 4TπMs = 12 kG, perpendicular coercivity Hc(⊥) = 5 kOe, and perpendicular anisotropy field Hk(⊥) = 23 kOe. These characteristics are easily controlled by changing the film composition. The perpendicular magnetic anisotropy field and the coercivity of the Co-Pt-B-O alloy films have close correlation with the film microstructure. To clarify this relation, the individual effects of oxygen and boron in the Co-Pt binary alloy film were investigated. The addition of boron makes the average grain size smaller, whereas the column diameter remains unchanged. The addition of oxygen to the Co-Pt binary alloy reduces the column diameter to about 5–10 nm, whereas the columns are densely packed without any gaps. The sole addition of oxygen or boron neither increases the perpendicular anisotropy nor the coercivity. The high perpendicular anisotropy and the large coercivity are derived from the fine columnar structure of 5–10 nm in diameter coupled with the inter-columnar gaps of about 1 nm and are obtained by the co-additions of oxygen and boron.