An analytical model is proposed to predict the electromechanical response of anisotropic piezoelectric composites containing spherical piezoelectric nano-inclusions under a uniform electric field. The electroelastic surface/interface theory for nano-size structures is introduced to analyze the nano-size effect. The analytical solutions for the elastic displacements and electric potentials in the nano-composites are derived exactly with consideration of the elasticity, permittivity and piezoelectricity of the interface. The unknown coefficients are determined by satisfying the boundary conditions with interface effect. Numerical examples show that the elasticity, permittivity and piezoelectricity of surface/interface express different effects on the stress and electric displacement in the nano-particle and matrix. The jump of stress and electric displacement under different material properties of the interface is also examined.