Abstract: The influence of the pipe diameter on the determination of grease stain thickness in oil pipeline using the fast neutron transmission method and scattering method was reported. The experimental device consists of the neutron source, neutron detector, and the compute based multi-channel analiser. In transmission method， the results showed that the sensitivity of determination of pipeline grease stain decreased with the increase of the pipe diameter，and the average measurement accuracy was 0.698 4 mm. In the scattering method, the exponential fitting was used, in data processing, and the results showed that linear interval increased with the pipe diameter, The Linear fitting results showed that the average R² was 0.958 3, and the measurement accuracy was 0.625 7 mm.