This study investigated the gas sensor fabricated with TiO2 added with metal oxide and catalyst to detect the main mixed gases, usually generated when the meat begins to decay, in order to measure the change in freshness according to protein denaturation. In particular, when fish begins to decompose, TMAO (trimethylamine oxide) is transformed into TMA (trimethylamine) by enzymes to regulate the salinity in fish, which is the major cause of fishy smell. The thick-film semi-conductor gas sensor for trimethylamine and ammonia mixed gas was fabricated with WO3-TiO2 prepared by sol.gel and precipitation methods. The nanosized TiO2 was mixed with WO3 and doped with transition metals (Pt, Ru, Pd and In). Particle sizes, phases and specific surface areas of sensor materials were investigated by SEM, XRD and BET analyses. The metal-TiO2 thick films were prepared by screen-printing method onto Al2O3 substrates with platinum electrode. It was shown that the highest sensitivity and selectivity of the sensor for trimethylamine and ammonia mixed gas by doping with 1 wt.%In and 15 wt.% WO3 was reached at the optimum operating temperature of 350 ℃.