Optical nonlinearity of thiourea and its metal complexes (Zn and Cd) in water is studied in the regime of low laser power using a continuous wave diode pumped Nd:YAG laser (532 nm, 50 mW). The Z-scan technique was used to measure the nonlinearity of the medium and it is found that the predominant mechanism of nonlinearity is thermal in origin. The nonlinear refractive index and nonlinear absorption coefficient of the samples are found to be in the order of 10 -8 cm2/W and 10-3 cm/W respectively. The inclusion of metal has improved the optical nonlinearity of the samples than its parent molecule. The third-order nonlinear susceptibility was found to be in the order of 10-6 esu. The self-defocusing effect observed was used to demonstrate the optical limiting action at 532 nm using the same cw laser beam. © 2010 Elsevier B.V. All rights reserved.