The self-focusing, thermal self-defocusing and self-phase modulation of argon ion laser in colloidal CdS semiconductor microcrystallites are first reported in this paper. The large nonlinear optical Kerr coefficient, n2, is measured, and is found to be ～105 times the value for CS2. The differences between nonlinear self-phase modulation and thermal self-phase modulation in the colloidal semiconductor miorocrystallites, which are of weak absorption, are analysed.