The water activity of millet flour was determined by way of Conway’s dish diffusion method.The results showed that the standard deviation of experimental data was smaller and have high accuracy when using quaternary systems being composed of saturated solution of magnesium chloride,potassium carbonate,sodium chloride and potassium chloride.Water activity of millet flour increased as the particle size decreased,but the extent of increases was relatively small.At 25 ℃,after 2 hours of water exchange equilibrium,it was determined that the water activities were 0.590,0.599,and 0.605,respectively for the particle size of 80 mesh,100 mesh and 120 mesh of millet flour.