The present paper deals with experiments and a prediction method for the void fraction of R134a vapor-liquid two-phase flow in horizontal smooth and microfin tubes in adiabatic condition. The void fraction is measured by the quick closing valve method. The smooth tube tested is 1024 mm in length and 7.52 mm in inside diameter. The microfin tube tested is 1015 mm in length and 8.86 mm in mean inside diameter; the fin height is 0.18 mm, the helix angle of fins is 25degrees and the total number of fins are 70. The experiments were carried out in the range of vapor quality from 1% to 96%, where the pressure was kept at 1.2 and 0.8 MPa and the mass flow rate was kept at 20 and 40 kg h(-1). It is confirmed that the void fraction for smooth tube is well correlated by the Smith or the Baroczy correlations. It is also shown that the void fraction in the microfin tube is lower than that of smooth tube in any quality and the prediction results using previous correlations for smooth tube are higher than the present experimental data of microfin tube. The void fraction prediction method consisting of a stratified-annular flow model and an annular flow model is proposed. In the stratified-annular flow model, it is assumed that most of liquid flows at the bottom of the tube and all grooves are filled with additional liquid. The momentum equations are constructed for regions of vapor, main liquid flow at the bottom and additional liquid flow in grooves, respectively. These coupled equations are solved numerically. In the case of annular flow model, it is assumed that all grooves are filled with liquid uniformly. The momentum equations in the vapor and liquid flows in grooves are also solved numerically. The values of the predicted void fraction are in good agreement with experimental data in high vapor quality region, while the predicted values are slightly smaller than experimental ones in low vapor quality region. (C) 2004 Published by Elsevier Ltd.