Liquid film thickness is an important parameter for predicting boiling and condensation heat transfer coefficients in a microscale slug flow. In the present study, liquid film thickness of oscillating slug flow in a capillary tube is experimentally investigated under adiabatic condition. Laser focus displacement meter is used to measure the initial liquid film thickness. Circular tube with inner diameter of 1 mm is used for the test tube, and water and ethanol are used as working fluids. Measurement is carried out using a capillary tube with one open end and the other connected to a stepping motor. Driving frequency is ranged from 1 to 10 Hz at equivalent slug stroke of 31.7 mm and 51.2 mm. As the frequency and equivalent slug stroke are increased, the liquid film thickness deviates from that in the steady condition and becomes thinner or thicker under flow acceleration or deceleration, respectively. The empirical correlations for the initial liquid film thickness under acceleration and deceleration conditions proposed in the previous study (Youn et al., 2015, 2016) well predict the liquid film thicknesses of the oscillating flows within 15% accuracy. (C) 2018 Elsevier Ltd. All rights reserved.