Abstract: Beryllium is an important material as neutron reflection layers in nuclear reactors. The size change of beryllium after long term neutron irradiation has a great influence on the reactor safety. To obtain the size change of beryllium assemblies after long-time irradiation for the assessment of service performance, a set of special tools were designed and manufactured. The size change of post-irradiation beryllium assemblies was examined on a three coordinate measuring machine. The measurement results indicate that beryllium assemblies of the SPRR-300 reactor have excellent stability after 29 a irradiation, even under the maximum neutron fluence of 6.78×10²¹ cm⁻². The section dimension has little change in local part of beryllium assemblies and the largest change is about 0.13 mm, which indicates that irradiation creep is the main reason for the dimension change of beryllium assemblies during the long-time service.