The effect of nonuniform microstructure on magnetic switching behavior for CoCrPt–SiO2 single layer media with oxygen contents (OCs) from 4% to 10% and recording layer thickness (tmag) from 2 to 27 nm is investigated. Plan-view transmission electron microscopy images clearly show nonuniform microstructure of CoCrPt–SiO2 with OC=10% along the film thickness direction: poor grain isolation at a thin layer, excellent grain isolation at a medium layer, and formation of subgrains at a thick layer. The highest coercivity (Hc) is not observed on the medium with the best grain isolation. An initial CoPt layer with less grain isolation forms on top of Ru interlayer with increasing OC in films. It exhibits domain wall switching, resulting in significant incoherent switching. This is consistent with large magnetic activation volume at tmag≤6 nm. Other plausible incoherent switching mechanism for thicker layer media showing low Hc is likely due to formation of subgrains with less grain isolation.