Fine powders of ZnO were synthesized by the sol-gel method. The shape of gel powders with calcination temperatures changed into the sheet structure, the needle shape, and the spherical grain. The growth rate of grain size was slow to 700$^{circ}C$ but high above 700$^{circ}C$. The bigger the grain size is, the higher the degree of crystallization is. The organic element in gel powders evaporated below 300$^{circ}C$. Temperature dependence of conductances showed the sigmoidal shape, but the temperature range of the constant conductances narrowed with the decrement of the calcination temperature of gel powders. The optimum sensing property for CO gas were observed with the specimen calcined at 500$^{circ}C$ and degraded with the increment of calcination temperature.