An iron powder for decomposition of organic chlorinated compounds, comprising from 0.03 to 0.5% by weight of Ni and from 0.005 to 5% by weight of carbon, wherein particles having a particle size of less than 53 .mu.m occupies less than 40% by weight, is used. It is particularly preferable that Ni, carbon and iron are partially alloyed. When the iron powder for decomposition is used by mixing with, for example, an Ni-free iron powder, the total Ni content can be reduced without deterioration of decomposition performance to organic chlorinated compounds. The iron powder has high decomposition performance in the treatment of a solid such as soil.