The nasal cavities are characterized by extremely complex three-dimensional structure. Modeling of airflow in the nasal cavities is important for evaluating nasal function such as olfaction, filtration, heat transfer and humidification. The aim of this study is to reveal experimentally the transitional nature in image-based realistic model human nasal airways by flow visualization. The inspiratory airflow pattern was visualized using the dye injection method and particle image velocimetry (PIV). As a result, significant differences were observed between flow in the right and left cavities, indicating strong dependence on the local geometry. Flow instability was first evident via oscillation of dye filament position in the upper and lower portion of the left cavity, whereas the flow was stable in the middle portion of the left cavity and in almost the entire portion of the right cavity. The high turbulent kinetic energy (TKE) values were observed in the free shear layer at the edge of the jet.