Quality change and fishy odor development in tilapia during freezing storage were studied. Conjugated diene( CD) contents increased with lipid oxidation occurring in fish muscle. Significant increase of total volatile basic nitrogen( TVBN) values at the storage of 9d indicated that microbial spoilage was started in the muscle. Degree of lipid oxidation could be represented by the value of thiobarbituric acid reactive substances( TBARS),which increased continuously through the whole storage,It sharply increased after 9 days. Results of sensory evaluation showed that grassy odor was detected in fresh tilapia muscle and the intensity decreased during storage,while intensity of offodours including fishy,rancid and putrid odors increased as storage time extended. Fishy and rancid odors of sample increased significantly after day 6. Within the storage,no putrid odor was found in tilapia muscle until day 9. Correlations between chemical indexes and sensory evaluation scores showed that the off-odour in fish muscle was mostly due to lipid oxidation. In the late period of frozen storage,microbiological spoilage was more severe.