In the current investigation, fabrication of Nickel nanopowders using mechanochemical process and the effect of milling time on particle size has been studied. Furthermore, the effect of using magnesium chloride as well as additional magnesium (in respect to stoichiometric ratios), as process control agent (PCA), on reaction progress and the size of Ni nanoparticles has been investigated. In this regard, planetary ball mill with maximum speed of 500 rpm was used. After mechanical milling to separate the Ni nanoparticles, the processed samples were leached in 10% HCl for 5 hours and rinsed with double-distilled water. Internal structural changes and morphological evaluations of powder particles during mechanochemical process were characterized by means of X-ray diffraction (XRD) and Scanning electron microscope (SEM). In mechanical milling of the stoichiometric powder mixture, continuing milling from 35 up to 40 h and further mechanical milling form 40 to 50 h results in a reduction and an increase in the particle size of Nickel nanopowder, respectively. Excessive Mg addition as PCA reduces the length of the reaction time comparing to the stoichiometric mixture. Moreover, the more the amount of Mg causes the finer the Ni powder particle. The addition of MgCl 2 as PCA to the initial powder mixture reduces the commencing reaction time comparing to milling of stoichiometric mixture of NiCl 2 and Mg powder (first stage). Also the addition of MgCl 2 as PCA causes a delay in finishing time of the reaction comparing to first stage and Milling the powder mixture with additional Mg (second stage). Excessive amount of MgCl 2 leads to the crystalline size reduction. The effect of MgCl 2 addition on controlling and reducing the crystalline size of Ni is greater than the addition of Mg
