Using the Z-scan technique and a 532 nm TEM00 mode laser pulse with a width of 2.8 ns and a waist radius of 15.0 mu m, we characterize photo absorption induced thermal lensing effect in a toluene solution of C-60. To fit the experimental results, we have to strictly solve the thermal acoustic wave equation, driven by the temperature rise, to achieve the solvent density change. Steady state (instantaneous expansion) approximation of this wave equation greatly breaks the agreement of the theoretical simulation with the experimental results. Accordingly, the experimentally observed thermal lensing effect is assured in the transient regime. (C) 2009 Elsevier B. V. All rights reserved.