Abstract: In order to provide a suitable ~(237)Np reference solution for nuclear fuel reprocessing and environmental science,a radioactivity reference solution of neptunium-237 in 0.5 mol/L HNO_3 is investigated.It is prepared by dissolution of a neptunium oxide stock from plant.The preparation procedure is as following: purification and assay for purity of neptunium,preparation of neptunium nitrate solution,making solution for ageing,subpacking the solution to ampoules,fire sealing,weighing for earch ampoule,homogeneity test,stability test,and certification.According to the statistical tools,the spot-check samples from the solutions in ampoule are measured for homogeneity test by 4πα(PC).The minimum spot-check sample size for measurement by 4πα(PC) is about 10 mg solution.The solutions in ampoule have been stored for one year,then the spot-check samples are measured for stability test by 4πα(PC),the results prove that the solutions are stable.The certified value is measured by five different laboratories using 4πα(PC),4πα(LS) and 2πα grid ionization chamber,and is verified by National Institute of Metrology.The radioactivity concentration of neptunium-237 is(1 702±35)(Bq/g,)(α=0.05).