A novel optical technology for on-line monitoring of DO (Dissolved Oxygen) is reported. As a DO sensor in water, Ruthenium (II) complex (Ru(ph<sub>2</sub>phen)<sup>2+</sup><sub>3</sub>) adsorbed on silica 60 gel particles emitted strong pink fluorescence steadily when a 470 nm wavelength blue LED (light-emitting diode) shined. The fluorescence could be quenched with molecule oxygen sufficiently; the higher DO concentration is, the weaker fluorescence intensity becomes, and the dynamic quenching processes were described by Stern-Volmer formula. Through reasonable correction and calculation, the concentration parameter of DO in water would be got. In laboratory, we used the optical sensor to measure DO concentration with oxygen-free water, oxygen-saturated water and natural water samples, and also researched the change of DO concentration in different temperatures. Experimental results show that measure range of the sensor is 0 ~ 20mg/L, and the accuracy is less than ±0.2mg. The sensor can be used on line monitoring of DO in water.