Void fraction of nitrogen gas-water two-phase flow in a circular tube from 1 to 10 mm in diameter was measured by neutron radiography. The measured void fraction results were compared with Ishii's drift flux model based on Mishima-Ishii's flow regime criteria. The void fraction was predicted well in annular flow region except for α<0.8. Mishima-lshii flow regime map was modified by a churn to annular flow transition criterion of α=0.8. The void fraction was predicted well by Ishii's drift flux model in annular flow region in every diameter tubes.