Abstract: The pressure drop in the narrow space channel is different from that in the regular size channel, and the bubble behavior in confined narrow space has great influence on the flow stability. A visual investigation on the gas-liquid two-phase flow was carried out in a narrow space channel with 2 mm in diameter under atmospheric pressure（0.101 MPa）. Experiments were carried out with different working conditions and gas-phase working fluids, including air, carbon dioxide and argon. It is found that the pressure drop increases along with the increasing of liquid mass flux and constant gas flow rate, and the corresponding interface morphology is elongated slug flow. However, the pressure drop decreases at first and then increases along with the gas flow rate with constant liquid mass flux, and the corresponding interface morphology is still elongated slug flow.