In recent decades vast areas of Bornean rainforest have been converted to monocultures, particularly to oil palm. This study describes and compares the diversity and persistence of small mammal species in fragmented forests within an oil palm plantation. Small mammals were trapped over 44 sampling days using mist-nets, harp traps, and cage traps. A total of 39 species of small mammals were trapped, represented by 22 volant (bats) and 17 non-volant species. These species were represented by 29, 15 and 26 species, and controlling for sampling effort by rarefaction, 19, 15 and 14 species of small mammals in high conservation value (HCV) forests, forest edge, and in oil palm plantation, respectively. Species diversity was greatest in the HCV forests (H’ = 2.22), followed by oil palm plantation (H’ = 1.11), and then edge (H’ = 1.07). The smaller HCV forest (116 ha vs. 990 ha) contained more species than the larger forest (25 vs. 17 species). We attribute this to differences in species relaxation rates and the possibility that species density compensation has occurred in the smaller forest. This study demonstrates the conservation value of small forest fragments in oil palm plantations to maintaining native species diversity and the ecosystem services they provide.