We suggested a simple method for obtaining the quantitative transition rate constants of trans-cis isomerization in the pseudo-stilbene-type of azobenzene molecule. The absorbances of the solution and film of a poly(malonic ester) containing disperse red 1(PDR1) were measured with the pumping beam intensity, and analyzed by the rate equation for the fractions of trans and cis conformers. The rate constants of PDR1 solution and film were successfully determined.