Abstract: The ultrafast excited state dynamics of trans-4-aminoazobenzene (trans-4-AAB) in ethanol was investigated by femtosecond transient absorption spectroscopy. After being excited to the S₂ state by 400 nm, trans-4-AAB decays from the S₂ state to the hot S₁state by internal conversion with time constant of ～70 fs. The photoisomerization through inversion mechanism on the S₁ potential energy surface and the internal conversion from the S₁ state to the hot S₀ state are observed, respectively. The average timescale of these two decay pathways is ～0.7 ps. And the vibrational cooling of the hot S₀ state of cis- and trans-4-AAB occur with time constants of ～4 and ～13 ps, respectively. Furthermore, the ultrafast intersystem crossing process are also observed. The timescale of intersystem crossing from the S₂ state to the T₄ state is about 480 ps while from the S₁ state to the T₂ state is ～180 ps.