We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co 90 Fe 10 films (20, 30, 40, and 50nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40nm-Co 90 Fe 10 -coated tip (50nm tip diameter) about a lateral detect density of 1200k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.