The carrier injection efficiency eta(i) of recently developed UV-B laser diodes (LDs) is estimated on the basis of fundamental calculations and measurements. A general procedure to estimate eta(i) based on the relationship between the differential external quantum efficiency and the cavity length proves to be inadequate because of the large variation. To estimate eta(i) without relying on the unstable output power, we analyze in detail how the threshold current density relates to the cavity length. By applying a light confinement factor of 3.5%, an internal loss of 10 cm(-1), a current density of 0.56 kA cm(-2) in the emission layers at zero gain, and a reflectivity of the mirror facet of 0.16, we estimate eta(i) approximate to 3.5% for UV-B LDs. This low eta ( i ) in UV-B LDs is due to unbalanced injection between electron and hole currents, which leads to electron overflow to the p-GaN side, as indicated by a simulation.