Abstract: In the present paper the reaction of hydrolyzed products of 1,3,5-trichloro-triazine (TCTA) with gelatin macromolecule was studied. When the hydrolysis proceeds under the conditions of 50℃, pH 9-10 and the duration less than 2 hours, the main product is 1-hydroxy-3,5-dichloro-triazine (HYDCTA), the two active chloro groups can react with gelatin macromolecule and form network cross-linking, leading to hardening of gelatin. When the hydrolysis duration of TCTA extended to 24 hours 1,3-dihydroxy 5-chloro-triazine (DIHYCTA) is the main product. DIHYCTA can not form network cross-linking with gelatin macromolecule but leaves the grafting reactivity toward the ammo-residuals contained in gelatin. When the graft reaction of DIHYCTA with gelatin is controlled at a favourite condition (50℃, DIHYCTA/gelatin 1:10-15, 24 hours), annew modified gelatin (DIHYCTA-grafted gelatin) can be obtained and used as coagulating agent for emulsion-making. The performance characteristics of this modified gelatin can be summed up as follows: 1. higher isoelectric point (4.6 pH) than PA gelatin. 2. shorter induction period to start coagulation. 3. clear separation of gelatin silver halidc micelle from free water phase. 4. higher re-dispersion and re-dissolution pH>6.0. 5. the main trouble with this modified gelatin is the relatively longer time required for thorough redispersion, in general 20 minutes is the minimal at 50℃ and at highspeed stirring.