This paper has reported a Monte Carlo simulation for vertical focusing monochromator Ge (511) of neutron diffraction residual stress instrument. The factors which will influence performance of instrument
such as the height of the monochromator and the angle between two crystals
have been investigated. Also the two conditions with the vertical focusing monochromator and with the flat monochromator which will affect the flux of sample table have been compared. At last the resolution of instrument is considered.