The ultra-fine silver halide particles were prepared in fish gelatin medium at the ratio of gelatin to silver as 4: 1 to 5: 1 by using double-jet precipitation method. Average size of particles, determined by TEM observation, increased from 16.6nm to 26.7nm as prolonging the addition time of the reactants from lmin to 9min or decreasing the addition rate of the reactants from 8mmol/min to 1.3mmol/min. A uniform size distribution was approached at a lower addition rate of the reactants. The sensitivity test of the ultra-fine AgBr/I particle emulsion sensitized by thiourea dioxide showed the increase in the sensitivity as increasing the amounts of thiourea dioxide added. However, the increase of sensitivity was associated with the increase in fog density. The sensitometric properties of the emulsion at low exposure intensities for long exposure times were also explored. The exposure required for the production of density (fog+0.1) in the thiourea dioxide-sensitized emulsion containing ultra-fine AgBr/I particle reduced as prolonging exposure time at low exposure intensity.