cis,cis-1,2,3,4-Tetraphenylbutadiene (TPBD) exhibits aggregation-induced emission (AIE) in the UV-blue band: the photoluminescence (PL) quantum yield of TPBD aggregates can differ from that of molecularly dissolved species by 2 orders of magnitude (>200). When the isolated molecules in solutions are cooled to extremely low temperature, they also emit intense light comparable to that in the solid state. TPBD thin layer shows on-off fluorescence switching behavior that can be utilized for the sensing of organic vapors. The phenyl substituents in TPBD are twisted in the solid state, and excimer formation is greatly prohibited. The cooling-enhanced emission of the TPBD solution and the fluorescence switching behavior suggest that the aggregation-induced emission is caused by restricted intramolecular rotation of the phenyl groups. The intramolecular phenyl rotations of TPBD can be regarded as rotational relaxations around their equilibrium positions, from which mean relaxation time is defined based on an Arrhenius equation. All the PL behaviors of TPBD can be well explained qualitatively by the magnitude of the relaxation time.