We performed the transient absorption measurement and the first rate equation (RE) analysis for cis isomer of 4-carboxy-2',6'-dimethylazobenzene to clarify the quantitative difference between the photoisomerization process and the thermal relaxation process from the S-1(C) excited state. The RE analysis enabled us to determine the cis-to-trans photoisomerization rate per each pump pulse to be 3% under the condition of the 430 nm, 150 fs pump pulse with energy of 200 nJ. Moreover, the signal due to the yielded trans molecules appearing in the transient absorption was assigned from the following observed result: the transient absorbance change at the 380 nm probe mostly decreased within 300 fs after the 430 nm pulse pumping and then slowly decreased to zero, while the absorbance change at the 350 nm probe had a positive constant component in the over one picosecond time region. The RE analysis showed that this constant component is due to the yielded trans molecules, and its positive value is due to the fact that the absorption cross-section of the S-0(T)-to-S-2(T) transition in their transmolecules is larger than that of the S-0(C)-to-S-2(C) transition in the original cis molecules. (C) 2009 Elsevier B. V. All rights reserved.