**Nakahara**,-Yasuaki; Tsutsui,-Tsuneo (Japan Atomic Energy Research Inst., Tokai, Ibaraki. Tokai Research Establishment). A computer code system NMTC/JAERI has been developed in order to perform Monte Carlo simulation calculations of nuclear reactions (nuclear spallation, fission, evaporation) caused in a heterogeneous medium by incident particles (nucleons such as proton and neutron, and pions) from an external source such as an accelerator and subsequent transport of particles emitted from nuclei. The base of NMTC/JAERI is a 1978 year version of NMTC, which was developed at ORNL and revised later at LANL and BNL. In the JAERI version a fission reaction based on the statistical model has been incorporated as a process in competition with an evaporation process. The mass number A of nuclides which can be included in a target/blanket has been extended from [A = 1; 8 <= A <= 239] to [A = 1; 6 <= A <= 250]. Charged particle transport in the medium is treated by the use of a continuous slowing down model to take into consideration of ionization loss. A conventional Monte Carlo method for neutron transport is applied to neutral particles. When a collision between a particle and a nucleus has taken place, nuclear reactions are calculated by a Monte Carlo method simulating a multi-step process of intra-nuclear nucleon cascades, fissions and evaporations. Calculated results for all the events in the medium are stored on magnetic files. Values of various nuclear parameters of the medium are obtained by analysing and editing the records on the files. Energy range of incident particles to which the code system can be applied is between 15 MeV and 3.5 GeV. But for pions the upper boundary is 2.5 GeV. NMTC/JAERI is used for the calculations of initial nuclear characteristics of nuclear fuel production and radioactive waste transmutation systems, and also useful for the design calculations of strong spallation neutron source, accelerator shieldings and even space ship shieldings.