The aim in this work is to determine event by event the counting, the distribution of energy, the distribution of momentum, and the angular distribution of the particles produced in the main nuclear reaction that takes place in neutron capture therapy by boron (BNCT) [10B (n, α)7Li]. In addition to a brief historical review, the results obtained from the computational simulation with Geant4 of the nuclear reaction that takes place in the BNCT are presented.