This paper examines experimentally the burning behaviors of thin-layer transformer oil on a water layer. A series of transformer oil pool fire experiments with different initial fuel thicknesses was performed. The burning process, burning rate, liquid temperature and radiative heat flux were measured. The experimental results show that the whole process can be divided into five typical phases: (1) rapid growth burning, (2) steady burning, (3) short boilover burning, (4) continuous boilover and (5) fire decay. The appearance of the middle three phases (2-4) depends on initial fuel thickness. The steady burning rate is found to be independent of the fuel thickness but slightly higher than that of burning without a water layer, because of the decrease in the boiling point of the fuel-water mixture. A critical fuel thickness was found to determine whether continuous boilover or short boilover will occur. The results also show the intensity of the initial boilover increases with increasing fuel thickness but the intensity of subsequent boilover of thin-layer burning can surpass that of thick-layer burning for some cases. A linear correlation was found between the time to boilover and fuel initial thickness in agreement with literature findings. The present results are of practical importance in situ-burning and thermal hazard analysis for fire accidents due to leakage of liquid fuels on water. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.