Two neutron activation analysis systems were designed and studied. Americium-beryllium (Am-Be) was employed as a neutron source in both systems. Two different types of neutron sources (which differ in neutron flux) were used. One of them was used paraffin moderator which was theoretically simulated using Monte Carlo method by GEANT4 package program of objective oriented programming. Theoretical calculation results of neutron flux to the paraffin was compared with virtual water irradiators and these result showed the neutron flux distribution along the irradiator axes of water irradiator greater then paraffin irradiator. The thermal (under Cd) and fast (over Cd) neutron flux were calculated along the irradiation axis and compared to the experimental measurement. This comparison showed a good degree of agreement. This system (paraffin irradiator) was used to measure the copper content in some chemical compound and standard solutions after pre-concentrated by sulfide. Another system for prompt neutron activation analysis was optimized and used to measure the concentrations of mercury, cobalt and cadmium after their precipitation with H2S to concentrate them. The mercury content of various selected chemical compounds and environmental water sample after preconcentrated were determined. The optimization parameters of the measuring system were 0.2-0.7 MeV energy region, 15 min measuring time, and 3 x3 NaI(Tl) detectors. The detection limit was 10 ppm. The cadmium content of various selected chemical compounds and environmental water sample after preconcentrated were determined. The optimization parameters of the measuring system were 0.35- 0.8 MeV energy region, 10 min measuring time, and (3 x3 ) NaI(Tl) detectors. The detection limit was 2 ppm. The cobalt content of various selected chemical compounds and standard solution after preconcentrated were determined. The optimization parameters of the measuring system were 0.35- 0.8 MeV energy region, 10 min measuring time, and (3 x3 ) NaI (Tl) detectors. The detection limit was 100 ppm.