The interaction between ZnO ultrafine particles and naphthothiacarbocyanine (NTC) dye in ethanol was investigated by monitoring the variations in intensity of visible fluorescence of ZnO with increasing concentration of NTC at different temperatures. It was found that the presence of small amounts of water in the solution greatly affects the aggregation state of NTC and thus the fluorescent behavior of ZnO. In the absence of water, a dynamic quenching was observed by NTC dye when its concentration was less than 7 mu M and 13 mu M at 20 degrees C and 42 degrees C, respectively. But at higher concentrations, the severe aggregation of NTC resulted in a great decrease of NTC dye concentration in solution, leading to a decreased quenching ability of NTC. When the quencher was dissolved in 95% ethanol(in the mixed ZnO-NTC solution, final water content 0-0.5%), aggregation of NTC did not occur. However, if the quencher was dissolved in water (final water content in mixed ZnO-NTC solution 1.5%-7%), due to the adsorption of water by ZnO, coagulation of ZnO particles occurred first, then a new aggregation state of NTC-containing water was formed on the ZnO surface, leading to a stronger fluorescence quenching.