Iron particles for purifying soil or ground water of the present invention comprise a mixed phase of .alpha.-Fe phase and Fe.sub.3O.sub.4 phase, and having a BET specific surface area of 5 to 60 m.sup.2/g, an Fe content of not less than 75% by weight based on the weight of the iron particles and a sulfur content of not less than 1,000 ppm. The iron particles are capable of decomposing or insolubilizing harmful substances such as organohalogen compounds and/or heavy metals, cyanogen, etc. contained in the soil or ground water in efficient, continuous and economical manners.