The Z-scan experiments were carried out to investigate the soluble carbon nanotubes. The thickness of sample is changed in order to study “thin” and “thick” sample respectively. The experiments are carried out using 8ns, 10Hz repetition rate laser pulses at 1064nm and 532nm. The transmittance curves of closed-aperture Z-scan show that nonlinear refraction is not the optical limiting mechanism of solubilized carbon nanotubes. The curves of opened-aperture Z-scan indicate that nonlinear absorption exists in this solution obviously, which may be the primary optical limiting mechanism of this material.