Flow regime, void fraction, rise velocity of slug bubbles and frictional pressure loss were measured for air-water flows in capillary tubes with inner diameters in the range from 1 to 4 mm. Although some flow regimes peculiar to capillary tubes were observed in addition to commonly observed ones, overall trends of the boundaries between flow regimes were predicted well by Mishima-Ishii’s model. The void fraction was correlated well by the drift flux model with a new equation for the distribution parameter as a function of inner diameter. The rise velocity of the slug bubbles was also correlated well by the drift flux equation. The frictional pressure loss was reproduced well by Chisholm’s equation with a new equation for Chisholm’s parameter C as a function of inner diameter.