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With pump-probe separated closed aperture Z-scan technique, nonlinear refraction can be detected and taken into account with transmittance signal of probe beam. Because the transmittance change induced by nanosecond pump pulse is temporal, temporal profile of probe pulse also changes with the thermal lens effect rising. In this study, a new concept for pulse shaping of nanosecond laser with thermal lens effect is discussed in terms of nonlinear optics. The rising of thermal lens effect in nanosecond regime is demonstrated by time-resolved measurement and theoretical model. Theoretical model that accounts for unsteady heat conduction and generation of acoustic wave in liquid sample shows good agreement with experimental results. As pump beam waist gets narrow, rising time of thermal lens effect becomes faster. It means drastic temporal change of transmittance is obtainable and controllable. Using this knowledge, temporal pulse-shaping of nanosecond laser pulse is tried in terms of pulse compression. It is shown that 7 ns pulse can be compressed by 30.0% in the present configuration theoretically. Also, experimental results show 32.2 ns pulse can be compressed by 21.7%.