The mass transfer rate of urea from a solid wall into a gas-liquid two-phase flow fluid was investigated under various liquid and gas velocities. The two-phase flow pattern in a small diameter tube was also visualized by using a transparent tube. Experimental observation showed that the two-phase flow pattern in a 6 mm diameter tube was slug flow under the experimental flow condition; i.e., both the superficial liquid velocity and superficial gas velocity were varied from 0.05 to 0.20 m/s. The length of the gas slug increased with the increase in the gas velocity. Experimental results also show that the mass transfer rate under two-phase flow operation was significantly higher than that under single liquid-phase operation. In the two-phase system, the mass transfer coefficient was proportional to the gas injection ratio at a specified liquid velocity. A quadric relationship between the mass transfer coefficient and the gas slug velocity is proposed in this work.