Streaming electrification has been identified as one of the main failure reasons for large power transformers. Different measurement methods including pipe flow method, rotating disk method and mini-static tester have been proposed to investigate this phenomenon. In this paper, a small scale laboratory based pipe flow model is established. The streaming electrification characteristics of a conventional mineral oil and a synthetic ester liquid were compared. Parameters including flow velocity, pressboard condition and oil aging were studied. The results indicate that the ester liquid exhibits a much higher streaming current than the mineral oil at the investigated flow velocities. Aging of the oil facilitates the charging process in the mineral oil but decreases the streaming current in the ester liquid.