The annular transition in the rod bundle is interesting because the small gaps between rods exist in the flow area. This is a very important phenomenon in the boiloff accident of nuclear reactor core. As a first attempt, the effect of small gaps in the flow area was studied by using the vertical rectangular ducts with different narrow gaps (2×100, 5×100, 10×100 mm). Based on the experimental results, the transition void fraction was defined and the transition model was proposed. The model gives a good prediction of the wide range of previous experiments including the data taken in the channels with small gaps.