Magnesium (Mg) is naturally found in bone tissue and is essential to human metabolism. It is well known that Mg ion improved bone-like apatite nucleation and growth. In this study, Mg ions were ion exchanged onto poly acrylic acid (PAA) thin film containing the carboxyl groups deposited on the commercially pure titanium (CP-Ti) surface using a plasma polymerization at discharge power 50 W for 5 min. Surface morphology and chemical composition of all samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Bioactivity of the Mg ion exchanged CP-Ti samples was evaluated by immersing in simulated body fluid (SBF) and MC3T3-E1 cell proliferation. The bone-like apatite forming ability was significantly influenced by Mg ion concentrations. Mg ion promotes bone-like apatite nucleation and growth on CP-Ti surface in SBF solution and improves MC3T3-E1 cell proliferation. It is therefore expected that CP-Ti and Ti alloys having a high biocompatibility can be obtained by ion exchanging the Mg ion after PAA plasma modified Ti surface. (C) 2012 Elsevier B.V. All rights reserved.