Concentrations of 38 elements in an environmental reference material Sargasso (NIES No. 9) prepared by the National Institute for Environmental Studies of Japan (NIES) were determined by instrumental neutron activation analysis (NIAA). The Sargasso samples (ca. 150500 mg) were irradiated by thermal neutron without cadmium filter and epithermal neutron with cadmium filter at Musashi Institute of Technology Research Reactor (MITRR). The activated samples were measured by three methods; conventional gamma-ray spectrometry using a coaxial Ge detector, anticoincidence counting spectrometry and coincidence counting spectrometry. Concentrations of 31 elements (Na, Mg, Al, S, Cl, K, Ca, Sc, V, Cr, Mn, Fe, Co, Zn, As, Br, Rb, Sr, Ag, Sb, Te, I, Cs, Ba, Ce, Sm, Eu, Tb, Yb, Au, U) were determined by the conventional combination of thermal neutron irradiation without cadminum filter and the gamma-ray spectrometry method. Concentrations of other 7 element (Ni, Se, Zr, Mo, La, Hf, Th) were determined by other combinations of irradiation and spectrometry method.