The field emission characteristics of an oxidized porous polysilicon were investigated with different Al2O3/Ti/Pt thickness. The Al2O3/Ti/Pt emitter showed stable electron emission characteristic compared with the conventional Ti/Pt electrode. The Al2O3 layer efficiently blocked the diffusion of emitter metal, which resulted in more reliable emission characteristics. Al2O3/Ti/Pt electrode thickness at 2nm/2nm/7nm showed the highest emission efficiency of 0.37% at Vps-17V. The investigated field emission life time of samples also demonstrated their stability over 40 minutes.