Hydraulic power rabbit in the Japan Materials Testing Reactor (JMTR) was used for highly sensitive neutron activation analysis of thorium in silica and aluminum. The thermal neutron flux of JMTR is higher than 1x10^n/cm^2/sec. A number of radioactive isotopes were made from impurities of high purity aluminum and silica irradiated in JMTR. ^Ta, which was produced from ^Ta by (2n, γ) reaction, was observed and made the determination of thorium difficult. The objective activation products ^Pa for the determination of thorium was isolated from majority of interfering radioactivity by a method combined with anion exchange and LaF_3 coprecipitation. It was found that the detection limit of thorium was less than 2x10^g. The method is useful to the analysis for thorium in high purity aluminum and silica that might be used to make very large scale integrated circuit
