We demonstrate 394 nm ultraviolet amplified spontaneous emission (ASE) with a low pumping power threshold of Eth = 1.3±0.2 μJ/cm2, from a thin film of an organic semiconductor 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) under the pulse excitation of a N2 gas laser (337 nm). 3-methyl and 3,6-dimethyl substituted CBP derivatives also exhibited pronounced ASE in the deep-blue region of 401 and 406 nm and ASE thresholds of less than Eth = 2 μJ/cm2. We also examined the ASE characteristics of N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD), N,N,N′,N′-tetraphenylbenzidine (DPABP) and N,N′-di(α-naphtyl)-N,N′-diphenylbenzidine (α-NPD). While TPD and DPABP showed low ASE thresholds, α-NPD did not show any ASE. We show that the large radiative decay rate (kf) of DPABP and TPD, which is derived from their short fluorescence lifetime (τf) and large quantum efficiency (ηf), leads to a low ASE threshold. On the other hand, the lack of ASE from α-NPD is ascribable to the small kf of 0.8±0.1×108 s−1, which is due to the rather long τf of 3.5 ns and small ηf of 0.29±0.02. In addition, we observed that the ASE gain is closely correlated with kf. A larger kf resulted in a larger ASE gain. © 2004 American Institute of Physics.