Abstract: The measurement of grease stain thickness at the internal wall of oil pipeline plays an important role in ensuring the safe operation of oil pipeline. In this paper, the thickness of grease stain in oil pipeline was measured by using the scanning technology combined with γ-ray transmission principle. Firstly, a set of grease stain thickness measurement device was developed by using the Cs-137 γ-ray source and NaI detector. Secondly, the Monte Carlo calculation software MCNP was adopted to perform numerical simulation analysis on this device. And thirdly, a case study was carried out on the medium-small oil pipeline with the diameter of 100 mm, thickness of 5 mm and grease stain thickness of 10 mm. The pipelines with and without grease stain were scanned respectively by using the collimated γ-ray beam to analyze the transmittance of γ-ray at the energy of 0.662 MeV. It is shown that the γ-ray scanning transmission method can be used to measure pipeline thickness and grease stain thickness. The research results provide the technical support for improving the oil transportation capacity of pipelines.