Aimed at improving the performance and compactness of the heat pipe heat exchangers, different forms of double-pipe heat exchanger units with densely longitudinal fins are obtained by metal additive manufacturing method. The manufacturing deviation is summarized, which shows that the internal structure is complete. The optical measurement shows that the roughness distribution on both sides of the inclined fin is not uniform. The steady state experiments are implemented to investigate the heat transfer characteristics of four densely longitudinal fins enhanced compact double-pipe heat exchanger units. The configurations are classified into two categories of straight fin type (DPHE-1) and periodically arranged fin type (DPHE-2). Within the Re ranging from 2000 to 15000, the tested results show that, due to the increase of heat transfer area and the enhancement of flow mixing effect, the DPHE-2 gives 1.3~1.4 folds averaged Nusselt number and 12.6~28.6 averaged friction factor than the DPHE-1. Comparing their overall heat transfer performance, the long fin scheme in DPHE-1 type performs best.