The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell .
With the best fitted physical parameters for GRB 990123 ( Panaitescu \& Kumar 2001 ) and the assumption that the parameters in the optical flash are the same as those in the afterglow , we show that : 1 ) the shell is thick but not thin , and we have provided the light curve for the thick shell case which coincides with the observation ; 2 ) the theoretical peak flux of the optical flash accounts for only 3 \times 10 ^ { -4 } of the observed .
In order to compensate this divergency , the physical parameters the electron energy ration and the magnetic ratio \epsilon _ { e } , \epsilon _ { B } should be 0.61 , 0.39 respectively , which are much different from those in the late afterglow .