Two-phase flow pressure drops of air-water in small horizontal smooth tubes of 1.02, 3.17, 5.05, and 7.02 mm are reported. The data are compared with homogeneous, slug, and annular flow models, as well as the commonly used empirical correlations. The tested results indicate that the empirical correlations available in the literature failed to predict the data. The slug flow and annular flow models give fair predictions with their corresponding flow regime data. The homogeneous model shows better predictive ability as compared to other empirical correlations. However, overpredictions of homogeneous and annular models are observed for most of the 1.02- and 3.17-mm data. Thus, modifications of the homogeneous model and the Friedel correlation are made by introducing the Bond number, Weber number, and other significant dimensionless groups to correlate the test data. The modified Friedel correlations and the modified homogeneous model show good agreement with the present data, as well as with five available data sets of ammonia and refrigerants from the literature.