Three novel dmit2- complexes: [(CH3) 4N][Co(dmit)2],[(C3H7) 4N][Co(dmit)2] and [(C4H9) 4N][Co(dmit)2], (abbreviated as MeCo, PrCo and BuCo, respectively and dmit2-=4,5-dithiolate-1,3-dithiole-2-thione), were synthesized and the third-order optical nonlinearities of their acetone solution of the same concentration were characterized by Z-scan technique at the wavelength of 1064 nm with laser duration of 40 ps. Their Z-scan curves all revealed a negative nonlinear refraction and a positive absorption at 1064nm.The molecular second hyperpolarizability γ of MeCo, PrCo and BuCo are 5.11×10-31,4.99×10-31,5.60×10 -31 (esu), respectively. The linear absorption coefficients a and the nonlinear absorption coefficient β of them are 0.3, 0.25, 0.32 (cm -1) and 1.53×10-12, 1.38×10-12, 1.85×10-12 (cm/W), respectively. The experiment results suggested that complexes with stronger linear absorption have larger nonlinear absorption and two-photon absorption. This difference is due to near resonant effect of sample with stronger absorption. So the linear absorption of complexes at 1064nm is a significant factor to influence the nonlinear absorption. These samples' large nonlinear absorption suggests that these materials have potential application in optical limiting.