A heat exchanger with small-diameter refrigerant tubes is disclosed. The heat exchanger has a plurality of air guide fins assembled with each other by one or more vertical rows of refrigerant tubes passing through the air guide fins. In the heat exchanger, each of the refrigerant tubes is a small-diameter tube having an outer diameter of not larger than 6 mm. In addition, four rows of offset surfaces are vertically formed on each of the air guide fins at a position between two tubes of each vertical row of refrigerant tubes through a pressing process such that the four rows of offset surfaces are arranged along a transverse direction of the fin. Four rows of vertical slits are each formed by two air guide openings defined between opposite side edges of each of the offset surfaces and the land surface of the air guide fin. In the heat exchanger, the number of the slits is reduced, in addition to changing the shape and dimension of the slits so as to allow the slits to be compatible with the small-diameter refrigerant tubes. The heat exchanger is also reduced in its production cost, accomplishes the recent trend of compactness, and minimizes its air-side pressure loss, in addition to accomplishing an improvement in its heat exchange operational performance due to its enhanced heat transfer efficiency. This heat exchanger is also improved in its productivity.