The best size of metal microwave absorbing particles was investigated.From the view point of skin depth and the critical size of single magnetic domain particle,the best size of metal absorbent is analized and numerically calculated.It is found out that for pure Fe while the size is within the range from 16 nm to 1 μm,its properties are satisfactory at 2 to 18 GHz.In order to utilize the quantum size effect of nano-particles to absorb microwave,the calculation results indicate that the size is less than 22 nm at least.