Application of nano zero valent iron (NZVI) is an innovative technology for water treatment and soil/groundwater remediation. Among various NZVI synthesis methods, chemical reduction is widely used due to its simplicity and chemical homogeneity. However, the synthesis procedure had not yet been clearly standardized. In this study, the effects of reaction conditions such as reaction time and NZVI concentration on NZVI characteristics and reactivity were investigated. Nanocluster and whisker structures were obtained under fast reduction conditions. The particle size was dramatically decreased from 87.4 nm to 9.5 nm under short reaction time and high reductant concentration. Simultaneously, the BET surface area was increased from 8.4 m(2)/g to 45.4 m(2)/g. The reactivity of NZVI was evaluated by a nitrate reduction test. The experimental data were interpreted by pseudo first order kinetics with first order deactivation. The reactivity was increased in the direction of high reductant concentration and fast synthesis, although deactivation increased in the same direction. Among the 4 reaction steps for NZVI synthesis, the nuclei formation step was determined to be the key step in determining NZVI characteristics. Therefore, it is important to control the reaction time and concentration in order to maximize the efficiency of NZVI. (C) 2011 Elsevier B.V. All rights reserved.