Gas-liquid slug flow in horizontal pipes occurs frequently in industrial applications and its heat transfer characteristics are very important but complicated due to the intermittent flow structures. This paper aims at investigating the heat transfer mechanism of both air-oil and air-water slug flow in horizontal pipes. An experimental setup was fabricated to investigate the flow and heat transfer characteristics of air-oil and air-water slug flow under cooling conditions in horizontal pipes. The test section was made of a copper pipe with I.D. of 26 mm and a polyvinyl chloride pipe with I.D. of 63mm using concentric geometry. The effect of flow parameters, such as liquid flow rate, gas flow rate, two-phase pressure drop multiplier, void fraction and slug frequency on heat transfer of gas-liquid slug flow was comprehensively investigated. The heat transfer coefficient of two-phase flow was significantly improved by the introduction of air and the maximum two-phase heat transfer enhancement ratio for air-water and air-oil flow were 150% and 300%, respectively. The circumferential temperature distribution and local heat transfer coefficient were also investigated. The local heat transfer coefficient at the bottom was around 25% higher than that at the top due to the uneven distribution of liquid and gas inside the pipe. Finally, a semi-empirical heat transfer correlation for gas-liquid slug flow was developed based on Reynolds and Chilton-Colburn analogies. The newly developed correlation predicts 96% of the present experimental two-phase heat transfer data within +/- 20% error and the mean relative deviations of the correlations are estimated to be -2.02% and +2.85% for Re-f <= 2300 and Re-f > 2300, respectively. This research helps researchers and engineers to better understand the heat transfer mechanism of gas-liquid slug flow. (C) 2019 Elsevier Ltd. All rights reserved.