Single phase and two phase flow characteristics in microsized glass tubes with i.d. (inner diameter) of 300 and 500 mm have been examined experimentally. Single phase pressure drop measurements are found generally in good agreement with Poiseulle flow theory. Transitional flow is found to start earlier at Reynolds number about 1600 as compared to the onset of transitional flow at Reynolds number of 2300 for macro-scale tubes. In addition, these glass tubes are employed for the investigation of adiabatic two phase flow characteristic by introducing gas phase via a stainless steel tube inserted at the center of the glass tube. Real time flow visualization obtained under the same flow condition are analyzed by both cross sectional void fraction (one dimensional drift flux model) and volumetric void fraction (image processing method). The analysis shows that the void fraction estimated by drift flux model (DFM) agrees with homogeneous correlation (a=b) and Armand correlation (a=0.833b). However image processing method seems to reveal that the slip ratio for the two phase flow is more significant and that the void fraction results are clustering between slip ratio of 3 and 7. Additionally, two phase frictional pressure losses are compared with the convention correlation for macro-sized tube (Lockhart-Martinelli model). It is found that measurements of the two phase frictional pressure drop can serve as a flow map to predict the flow patterns when the flow in the channel is not transparent.