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A beam shaping assembly (BSA) has been designed in the beam tubes of G.A Siwabessy reactor for BNCT application. The BSA is used to decrease thermal neutron, fast neutron, and gamma dose of neutron beam output. The design was carried out in a simulation using the Monte Carlo method with the PHITS computer code. Design of BSA was obtained with TiF3 materials of 100 cm as a neutron moderator and gamma filter, Al of 2 cm as a reflector, aperture and shield using Pb and LiF. This design was able to reduce fast neutron and thermal neutrons to 100000 times, gamma doses to 10000 times, while decreasing epithermal neutron to 10000 times. To meet BNCT requirements, the BSA design should be placed at a depth of 100 - 200 cm of the radial type of RSG-GAS beam tubes.