Photoluminescence spectral narrowing of 2,5-bis(4-biphenylyl)thiophene (BP1T) single crystals has been studied for various excitation lengths at 12 K to investigate the origin of its light amplification. In the range of excitation length L < 1.5 mm, the excitation energy threshold for the spectral narrowing of the 0–1 vibronic transition band decreases with increasing excitation length. This behavior is not consistent with that of stimulated emissions. The present results give a plausible interpretation that a cooperative emission process contributes to the light amplification of the 0–1 vibronic transition of the BP1T single crystals.