The third-order optical nonlinear susceptibilities χ(3) of coated Bi2S3 nanoparticles doped organically modified sol-gels under different thermal treatment conditions have been measured using the Z-scan technique with 38 ps pulse excitation at 1.064 μm. The χ(3) for the sample dried at room temperature is 5.8×10-13 esu, and for the sample heated at 150℃ for 2 h in a nitrogen atmosphere is 3.7×10-12 esu. It is clear that the χ(3) of the latter is larger than that of the former. Mechanism for the effects is discussed.