Cyclodextrin (CyD) effects on photodegradations of organophosphorus pesticides in humic water were monitored on the basis of the increase percentage of the photodegradation rate constant for CyD-containing humic water. Remarkable promotion effects of CyDs were observed in humic water for photo-induced radical generation. The promotion effects could be mainly assigned to the inclusion effects of CyDs to catalyze interactions of pesticides with reactive radicals generated by the humin photosensitizer and inclusion-trapped in CyDs.