Using intense pulsed laser beam of 5320 ? to excite the BDN dye solution inside the F-P interferometer, we have found the transient and intrinsic optical bistability effect. The various possible mechanisms to induce the refractive-index change of the sample medium and their relative contributions have been estimated; finally it is confirmed that in the duration of input laser pulse the refractive-index variation increases with time approximately in the form of semi-cubic parabola, and the main mechanism to cause this variation is the opto-thermal effect of the sample medium.