Abstract: The fully developed slug flow in a three-side heated narrow rectangular channel with a 3 mm×43 mm cross section was experimentally investigated by using deionized water as experimental medium in this paper. The visualization experiment of slug flow was carried out by using high speed camera. Four evolution behaviors of the slug flow were observed, which were the full development of slug flow, the appearance of sandwich-type slug flow, the behaviors of merging small slug into large slug and the behaviors of merging large slug into elongated slug. Effect of some thermal parameters on void fraction of the slug flow was analyzed. By introducing the Reynolds number, empirical correlation of void fraction of the slug flow suitable for the three-side heated narrow rectangular channel was obtained by nonlinear regression analysis for experimental data of slug flow. The results show that the new empirical correlation can accurately predict void fraction of slug flow in the three-side heated narrow rectangular channel. The relative error of predicting value is 12.36%.