We report the experimental results on the effect of pulse width on transient thermally induced optical nonlinearities in the nanosecond r egime. We have obtained the Z_scan curves of the CS2 solutions of nigrosin e for different pulse widths, and analyzed the co_existence of third_order and transie n t thermally induced optical nonlinearities. The process of changing from transi ent state to steady_state thermal_induced optical nonlinearities was observed. M eanwhile, we have obtained the results of numerical simulation by solving simult aneously acoustic and electromagnetic wave equations, which agree well with the experimental results.