Ba0.5Sr0.5TiO3/MgO (BST/MgO) composite ceramics have been prepared in situ by using an oxalate precipitation process. SEM and TEM images show that oleic acid modified BST/ MgO nanocomposite exhibits helminth-like particles consisting of BST core and MgO shell. Results reveal that oleic acid modification is chemisorbed on MgO particles as a carboxylate to form MgO shell, which connects to interparticles. Accordingly, the BST particles are well surrounded naturally by a thin MgO layer in the composite ceramics. As a result, the oleic acid modified BST/MgO composite ceramics shows better dielectric properties, with permittivity of 203, loss tangent of 0.0025 (at 2.4 GHz), and tunability of 10.2%.