The extinct layer of Si(311) perfect single crystal has been investigated by neutron diffraction method with the residual stress diffractometer DN1-M installed at the experimental hall of Indonesian multipurpose reactor RSG-GAS, in Serpong, which provides micro beam and point detector arrangement. A Si(311) perfect single crystal with thickness of 5mm was used in this experiment. The crystal was finely polished at one side and roughly polished at the opposite one. It was measured that diffracted beam from the finely polished side shows very low intensity with narrow peak profile, while for the roughly polished surface, shows stronger and broaden peak. The diffraction layer of rough surface was determined to be 0.36mm, while the smooth one was 0.28mm. It was also found that in the direction of crystal thickness in between layers no diffraction peak was detected. By these experiments it was confirmed that the applied stress on Si(311) perfect single crystal produced thicker diffraction layer. This technique is one that can be used in order to enhance the total diffracted neutron, which is desired to produce a good monochromator system.