The particulate behaviors of nickel ferrite were investigated under the simulated PWR shutdown chemistry conditions. Temperature of the simulated water with concentration of 0.1 ppm Li and 2,000 ppm B was dropped from 300 degrees C to 150 degrees C with a rate of 0.625 degrees C/min and then constantly maintained at 150 degrees C under the pressure of 2,500 psi. The on-line particle counting and the concentration measurement of nickel dissolved were performed under 5, 15 and 25 cc/kg H2O dissolved hydrogen. Experimental results showed that total particle count in the simulated water was not greatly changed for three hydrogen concentrations as temperature was decreased. However, particles were smaller as temperature was decreased and then maintained constantly when temperature was constant. The degree of variation in particle size distribution was greater at 15 cc/kg H2O dissolved hydrogen than any other dissolved hydrogen concentrations. Concentration of nickel ion was increased as temperature was decreased and was higher at 15 cc/kg H2O dissolved hydrogen than any other dissolved hydrogen concentrations. Theses results show that nickel ferrite is unstable with temperature variation and at dissolved hydrogen concentration of 15 cc/kg H2O.