The experimental determination of the thermal neutron diffusion length in gra- phite was carried out by measuring the distribution of neutron fiux along the x, y, and z axis. The neutron flux distribution have been measured with a BF,3 counter using a Ra - Be neutron source. For the calculation of the results the least sguares method was chosen in order to obtain the most probable value of the diffusîon length. The value of the diffusion length was found to be L = 58 i 2 cm for the Diffusi- on Stack and L — 53,46 cm (average) for the Exponential Stack.