The fin and tube heat exchanger with a semi-dimple pair is numerically studied in terms of the air side's thermal performance. The investigated parameters consist of the diameter, attack angle and the placed location of the semi-dimple. The results show that the 30 attack angle, 5.5 mm vertical distance and 7.5 mm horizontal distance can perform the highest j/f ratio or "goodness parameter" of every semi-dimple's diameter. Additionally, the thermal performance of the fin with a semi-dimple pair is also compared with that of the fin with existing semi-dimples and the plain fin within the Reynolds number ranging from 813 to 4019. Obviously, the existing semi-dimple exhibits the highest heat transfer coefficient and pressure drop. However, the semi-dimples' pair demonstrates a goodness factor of 33-37% greater than the existing semi-dimples and 15-20% greater than the plain fin, approximately. (C) 2017 Published by Elsevier Ltd.