A porous surface layer (a) was produced on ultrafine-grained Ni 50.8Ti 49.2 alloy (UFG NiTi) surface by sandblasting with HF/HNO 3 acid etching (AE-NiTi). This surface is bioactive and could yield apatite formation when soaked in simulated body fluid for 14 days (b). Osteoblast-like cells attached well after 4 h incubation (c1) and proliferated significantly after 3 days culture (c2). Such porous surface also presented lower corrosion rate and high pitting corrosion resistance in simulated body fluid compared with untreated coarse-grained NiTi (CG Ti) and UFG NiTi, as well as UFG NiTi with sandblasting (SB-NiTi) and UFG NiTi with combined sandblasting, acid etching and alkali treatment (AEAT-NiTi). Display omitted ► Irregularly rough, microporous and hierarchical porous surfaces were produced by sandblasting, acid etching and alkali treatment. ► Sandblasting with acid etching followed by alkaline treatment severely impaired the corrosion resistance. ► Sandblasting plus HF/HNO 3 solution etching increased the apatite forming ability. ► Sandblasting enhanced cell attachment and acid etching increased cell proliferation. Bulk ultrafine-grained Ni 50.8Ti 49.2 alloy (UFG-NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study, and to further improve its surface biocompatibility, surface modification techniques including sandblasting, acid etching and alkali treatment were employed to produce either irregularly roughened surface or microporous surface or hierarchical porous surface with bioactivity. The effect of the above surface treatments on the surface roughness, wettability, corrosion behavior, ion release, apatite forming ability and cytocompatibility of UFG-NiTi alloy were systematically investigated with the coarse-grained NiTi alloy as control. The pitting corrosion potential ( E pit) was increased from 393 mV (SCE) to 704 mV (SCE) with sandblasting and further increased to 1539 mV (SCE) with following acid etching in HF/HNO 3 solution. All the above surface treatment increased the apatite forming ability of UFG-NiTi in varying degrees when soaked them in simulated body fluid (SBF). Meanwhile, both sandblasting and acid etching could promote the cytocompatibility for osteoblasts: sandblasting enhanced cell attachment and acid etching increased cell proliferation. The different corrosion behavior, apatite forming ability and cellular response of UFG-NiTi after different surface modifications are attributed to the topography and wettability of the resulting surface oxide layer.