An experimental study on the detection of grease stain in 074 mm petroleum pipelines in Karamay Oil-field of Xinjiang was carried out by y ray transmission method. The response curve of grease stain thickness in the pipeline was measured to be In（N0 / N） = 0.00392d - 0.00677. We have also investigated the effect of thermal insulating layer. The experimental setup consists of a 137Cs γ radioactive source and a NaI （Tl） scintillation detector. Experimental results indicate that the thickness response of grease stain is sensitive and the thermal insulating layer has no too much effect on the obtained results.