Thin films of rhombohedral-tetragonal solid solution (1-x)BiFeO3-xBiCoO(3) (BFCO) with x = 0-0.30 were prepared by chemical solution deposition on SrTiO3(001) and LaAlO3(001) substrates. Thin films grown on SrTiO3 substrate shows a gradual second order change from a rhombohedral phase to a tetragonal phase with c/a = 1.04. The piezoelectric constant d(33) initially increased with increasing Co content, reached 80 pm/V at x = 0.05, and gradually decreased at x above 0.05. Films on LaAlO3(001) having smaller lattice parameter than SrTiO3 shows a distinct change from the rhombohedral phase to the tetragonal phase with a large c/a = 1.23. d(33) of films on LAO reached 100 pm/V at x = 0.10, almost double that of BiFeO3. Co substitution in the rhombohedral phase enhances the piezoelectric response.