The general design of a high-resolution diffraction beamline at a third generation X-ray synchrotron radiation source is presented.For this,we introduce the basics of high resolution diffraction and the optical elements necessary to prepare a nearly parallel but intense X-ray beam with well-defined photon energy for high-resolution application.In particular,the function of double-crystal and four-bounced crystal monochromators is explained in terms of X-ray dynamical theory.As an example,we present the layout of the High Resolution Diffraction(HighRes)beamline at the new synchrotron radiation source PETRA Ⅲ in Hamburg(Germany)which will become operational in 2009.By optimizing the optical components.a resolution in q-space will be achieved down to △q=10-5 nm-1with micron beam size and a flux of more than 10 11 cts/s.