Realization of compact and efficient air-cooled absorber is the key technology for the small capacity absorption refrigerator for domestic use. Vertical pipe with absorbent flowing inside and air flowing outside is the best choice for the air-cooled absorber due to easy addition of fins to enhance heat transfer of the air side. In this paper, firstly the modeling of the absorption process in the vertical pipe for constant heat transfer coefficient on the outer surface is described. Then, experimental results are presented for pipes of inner diameters 8∼26 mm, pressure of 5 and 6 mmHg and outer surface heat transfer coefficient of 2000 and 3300 W/(m2·K). It was found that for pipes with diameter of 13 mm or more, the absorption process is well estimated by the proposed model. The absorption with addition of surfactant is estimated by the pseudo turbulent method. The necessary heat transfer area of the air-cooled condition is about 3 times that for the wate-cooled condition.