In order to investigate the third-order optical nonlinearities of a novel type of organo-metallic compound, ［C16H33(CH3)3N］Ni(dmit)2 (dmit2-=4,5-dithiolate-1,3-dithiole-2-thione), abbreviated as CTNi, a sample solution with concentration 1.0×10-4 mol/L in acetone was prepared and its third-order optical nonlinearity was investigated using Z-scan technique at 1064 nm with 40 ps laser duration. Strong saturable absorption was found, the excited states effective absorption cross section was σeff=1.47×10-18 cm2, and the corresponding nonlinear absorption coefficient was β=-4.36×10-12 m/W. In addition, Z-scan curves revealed a self-defocusing effect with a large negative nonlinear refraction coefficient n2=-1.55×10-18 m2/W.